The purpose of this study is to examine student and teacher characteristics related to students' mathematics achievement. The study group for this research was composed of 1533 students who participated in the mathematics subtest of the transition from basic education to secondary education applied in 2016 and 36 mathematics teachers who teach these students. The data for the research are collected with the prepared student and teacher questionnaires. In the analysis of the data, two-level hierarchical linear models are used. According to the results, the most important teacher characteristic related to students' mathematics achievement is the level of professional satisfaction of the teacher. In addition, teachers' seniority, main field education, and classroom management skills have a significant and positive effect on mathematics achievement. Also, teachers' participation in professional development activities has a significant and negative effect on mathematics achievement, but this effect is small in practice. At the student level, the student's self-confidence in mathematics has the most important effect on the subject's achievement. The variables of students' liking mathematics and their anxiety towards mathematics have negative and significant effects on mathematics achievement, but these effects are small in practice.