Effective teaching practice for improving student achievement in mathematics is a critical area for instructional design. Further, results from international assessments of mathematics achievement have indicated that students in Korea typically earned test scores higher then international averages. The purpose of this study was to investigate the effects of mathematics beliefs and classroom instructional practices on the achievement outcomes of eight-grade students in Korea using data from the TIMSS 2007 assessment. There were several significant findings from this study. Students who earned high test scores frequently engaged in independent learning activities and decided on their own procedures for solving complex problems during their mathematics lessons. In addition, students who showed high achievement levels expressed positive self-beliefs about their abilities in mathematics. These findings indicate that it is important to simultaneously examine the effects of students beliefs and instructional practices when investigating factors associated with achievement outcomes.