This dissertation consists of three articles that explore the effect of professional development (PD) on teachers‘ understanding and implementation of Science, Technology, Engineering, and Mathematics (STEM) project based learning (PBL), and the effect of STEM PBL on students‘ mathematics achievement. Teachers in three high schools participated in the research activities. They attended sustained PDs provided by one STEM center based in a Southwestern university, and were required to implement STEM PBLs once every six-weeks for three years (2008 through 2010). The first article employed a mixed-method case study to explore the relation between the quality of the teachers‘ in-class STEM PBL implementations, understanding of the PBL in STEM education, and attendance in the STEM PBL activities. Quantitative findings indicate that attendance in the PD activities was significantly correlated with the quality of the in-class PBL implementation in 2010, yet not in 2011. Moreover, qualitative findings show that the teachers viewed the STEM PBL pedagogy as a means to promote student interest in mathematics, cultivate the interdisciplinary research culture in K-12 classrooms, and help improve students‘ content understanding. The second article investigated the effect of STEM PBL, especially on Hispanic and at-risk students‘ mathematics achievement. The participants were 528 students in the three STEM PBL high schools and 2,688 students in non-STEM PBL schools in the same region. Latent growth modeling was used to analyze the repeated measures across years. STEM PBL instruction positively influenced Hispanic students‘ achievement in mathematics, but not at-risk students. The third study investigated whether participating in STEM PBL activities affected students who had varied performance levels, and to what extent students‘ individual factors influenced their mathematics achievement. The participants were 836 high school students in the three schools. The findings from the hierarchical linear modeling showed that low performing students showed statistically significantly higher growth rates on mathematics scores than high and middle performing students, over the three years. In addition, student‘s ethnicity and economic status were good predictors of academic achievement. This dissertation is the first to reveal the effect of STEM PBL on student academic achievement relating to inservice teacher PD by employing the sophisticated research methodology.