Using data from the Programme for International Student Assessment (PISA), this study implements two statistical analyses to investigate the effects of student and teacher characteristics on students' mathematical achievement. First, the authors conduct an exploratory factor analysis to explore the factor structure for the various student and teacher variables of interest in this study. Second, they perform hierarchical linear modeling to analyze students' and teachers' multilevel structure in a school. The results suggest that student characteristics such as mathematics interest, instrument motivation, mathematics self-efficacy, mathematics anxiety, mathematics self-concept, and out-of-school study time predicted 39.9% of mathematical achievement variance. The results also suggest that mathematics self-efficacy had the largest effect on mathematical achievement. Teacher characteristics such as teacher-directed instruction, cognitive activation, teacher support, classroom management, and student-teacher relations predicted 34.9% of mathematical achievement variance. This study's results have implications for educators in fostering a positive learning environment to increase students' mathematics interest and self-efficacy, and focus on specific teacher characteristics to increase students' mathematical achievement.