This paper examines students' self-motivation in mathematics learning between girls and boys using secondary analysis of the Third International Mathematics and Science Study (TIMSS) conducted in 2003 by the International Association for the Evaluation of Educational Achievement (IEA) involving eighth graders. Components of self-motivation include students' self-confidence in learning mathematics, attitudes towards mathematics, the value students place on mathematics, students' initiative in looking up for information concerning mathematics topics and extra mathematics classes outside of classroom hours. The paper also examines the relationships between these components and mathematics average achievement between girls and boys.