The purpose of this study was to examine the effect of professional development of mathematics teachers on students' performance in mathematics. A total of 4498 8th grade students and 146 school teachers from Turkey participated in the Trends in International Mathematics and Science Study ( TIMSS) in 2007. The participants ( 4429) were those who completed all questions in the teachers' questionnaire related professional development and students participated in the mathematics test. Students' performances were measured in content and cognitive dimensions. Content areas are Number, Algebra, Geometry, Data and Chance; cognitive domains are Knowing, Applying and Reasoning. ANOVA was used to identify differences if any among mathematics performance of students based on their teachers' professional development experiences. The results revealed that professional development makes a big difference and effects students' performance in positive way. (c) 2011 Published by Elsevier Ltd.