This study aims to identify the most dominant factor of the teacher's role as a motivator that influences students' interest and motivation to perform in mathematics achievement. It is conducted in eighth grade of senior high school with 209 students, consisted of five state schools and two private schools from seven regencies in North Sumatera. The data collecting technique uses questionnaire about students' interest and motivation toward mathematics and teacher's role as motivator. Numerical data on mathematics achievement of students is obtained from school documents. The result of data with path analysis is obtained by dominant factor of teacher's role as motivator that is factor of delivery of learning goal and learning comfort equal to 6.10%, and 6.00% is influenced by the delivery of learning objectives and variations of learning approaches, 5.17% is influenced by the delivery of learning objectives, 5.06% is due to variations in the learning approach, 4.61% is influenced by learning comfort and variation of learning approach, and 4.26% influenced by pleasant class atmosphere.