Research studies have identified several factors related to mathematics achievement of students in Korea. Further, results from the Trends in International Mathematics and Science Study (TIMSS) assessments have shown that instructional practices and beliefs about mathematics were significantly associated with mathematics achievement of students in Korea. The purpose of this study was to examine the effects of engagement in mathematics lessons and confidence in mathematics on achievement test scores of students in Korea using data from the TIMSS 2011 assessment. There were several significant findings from this investigation. Students who held positive beliefs about their mathematics ability tended to show high levels of achievement. Conversely, students who expressed negative comparisons of themselves to other students were more likely to earn lower achievement test scores. Further, students who were engaged in their mathematics lessons also had higher test scores. These findings indicate that both mathematics beliefs and engagement in school lessons were significantly related to test scores for eighth-grade students in Korea.