The purpose of this study is to compare and contrast student, teacher, and school factors that are associated with student mathematics achievement in South Korea and the United States. Using the data from the Trends in International Mathematics and Science Study (TIMSS) 2011, this study examines factors that are linked to teachers who deliver quality instruction with high self-efficacy in both countries. We also investigate the association between teachers with high-quality instruction and high self-efficacy and 8th graders' mathematics achievement. It was found that teachers' perceived academic emphasis was commonly associated with teachers who claimed to provide high-quality mathematics instruction with high self-efficacy. However, the two countries' results differed in the association between teachers' opportunities to learn in professional development programs and high-quality instruction with high self-efficacy. Implications from this study suggest that the quality and training of teachers and students' gender gap in achievement are significant issues.