The aim of the study is to develop a web-based mathematics teaching (WBMT) material and to evaluate the effectiveness of the WBMT material for 9th grade students learning the concept of mathematical function. Firstly, a WBMT material was designed and piloted. As a result of this pilot study the site was revised as a final form. The study was conducted during the fall term of 2004-2005 academic years and was carried out in two different classes taught by the same teacher. Through mixed methods study the qualitative and quantitative data were collected from the sample, consisting of eighteen teachers and eighty 9th grade students. One of these teachers taught the control and experimental groups at the high school where the main study was conducted. This examination included comparing the results of students experiencing learning with WBMT with those do not, and interpreting the teachers' responses to the use of WBMT materials. The analysis of the data suggests positive effect of WBMT on student learning of mathematical function and on attitudes towards WBMT. However, the teachers all represented and shared some common ideas that because of the technical problems and readiness of teachers and students there would be some problems in terms of successful implementation of WBMT in schools. Nevertheless, the results provide support for the use of this WBMT material as a complement to traditional classes.