The purpose of this study was twofold. First, it aimed to determine whether the teachers' choice of motivation style influenced their mathematics teaching. Second, it investigated the influence of teachers' choice of motivation style on their willingness and ability to implement reform-based mathematics curriculum and instruction. Eight high school mathematics teachers from the two groups of autonomy supportive and controlling style of motivation were observed over the course of 5 months as they implemented a reform-based unit of instruction in geometry. All teachers worked in an urban school district. There were significant differences between the two groups of teachers relative to how they taught mathematics and interacted with students. Autonomy-supportive teachers' behaviors were more compatible with current reform recommendations for mathematics teaching. Moreover, autonomy-supportive teachers were more willing to teach and more successful in implementing the reform-based unit of instruction in geometry. All teachers experienced difficulty creating a student-centered mathematics classroom.