This study implements a teacher professional development program with an aim toward developing mathematics teachers' discourse-based assessment practice (DAP) and exploring its possible impact on teacher discourse in sessions and in DAP in the classroom. DAP is a type of formative assessment practice which consists of questioning and feedback. Teachers use questioning to probe students' learning progress and interpret students' responses to figure out their learning needs. Then teachers take informed actions, either statements or follow-up questions, to move student learning forward. A theoretical framework of teacher professional learning, a generative and recursive model, derived from review of the literature guided the implementation of the study. Two research questions are proposed in this research project: (a) How does engagement in professional development focused on DAP affect teacher discourse within the professional development community? and (b) What impact, if any, does the professional development program have on teachers' DAP in their own classrooms? In order to address these two research questions, this study takes a single and embedded case study design to explore the change of mathematics teachers' discourse within a learning community and to assess improvement of DAP in the classroom. In this study there are three junior high school teachers and one elementary school teacher who taught in schools with diverse socio-economic status (SES) participated in this study. The program was executed by adapting a teacher professional learning design, lesson study, that took place for more than six months. The implementation of this study consisted of three stages: (a) In the stage of lesson planning, participating teachers collaboratively made lesson plans according to lesson goals and teaching experience. They brainstormed in order to compile questions that could elicit learning evidence from students; (b) In the stage of lesson teaching, they executed these lesson plans and videotaped instruction in individual classrooms; (c) In the stage of lesson reflection, they met to watch videotapes or read lesson transcripts, to discuss teaching episodes, and to reflect upon their DAP. The researcher served as a facilitator who conducted and encouraged teacher discussion and addressed teachers' learning needs as they emerged from teacher discussion. Qualitative methods were applied to analyze teacher discourse and DAP. Research data includes transcripts of teacher professional meetings and lessons, as well as field notes. On one hand, teacher discourse in sessions was broken into idea units and encoded for data analysis. On the other hand, Mehan's (1979) conceptual framework was applied to organize lesson transcripts into the initiation-reply-evaluation/feedback (IRE/F) sequence. The analysis of teachers' DAP was carried out in extended sequences. Member checks were conducted to ensure the credibility of data analysis. The coding results are presented in tables that include the frequency and percentage of each code. After teachers' participation in the research project, several research findings are suggested: First, the focus of mathematics teacher discourse in sessions shifted from teaching techniques to students' thinking about mathematics. Students' mathematical thinking became the teachers' objects of inquiry (Crockett, 2002) and the core of instructional decision-making. The teachers formed a relationship of critical colleagueship and gave productive suggestions and critiques about their partners' DAP. Their discourse became more focused and deeper than before. The evolution of teacher discourse could be observed in the development of teachers' DAP. Secondly, teachers' DAP developed from convergent formative assessment to divergent formative assessment in the classroom. In other words, teachers' development of DAP evolved from examining whether students could or could not do something to exploring what students had learned. They were able to apply various types of questions to probe students' learning progress and to provide productive feedback for moving students' learning forward. The teachers began to apply multiple strategies of formative assessment practice to support their students' mathematics learning. Teacher professional learning could occur in the process of teaching, that is, teachers' understanding of student learning and teaching strategies were resulted from their interactions with students. Teachers who taught in low-SES schools seemed to benefit more from professional development of DAP than their high-SES counterparts. This group of teachers displayed the highest frequencies of reflecting on their own DAP--not only in sessions but also in the classroom. They also reported their awareness of improvement in DAP in sessions. SES seems to be a factor of context for teacher professional development. This study intended to contribute to mathematics teachers' professional learning and its impact was revealed in the evolution of teacher discourse in sessions and through DAP in the classroom. In particular, teachers who taught in low-SES schools benefited more from their participation in the teacher professional development program. Given the fact that the learning achievement gap of mathematics between high-SES students and low-SES students in Taiwan has been large, and low-SES students' learning benefit from their teachers' formative assessment practice (Black & Wiliam, 1998a), this finding is promising for developing teachers' DAP of low-SES students to narrow the achievement gap and thereby make a significant contribution to equity issues in Taiwan. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]