The purpose of this study is to examine the relationship between mathematics self-efficacy beliefs and sources of self-efficacy in two elementary school students whose mathematics achievement are at different levels. Case study design, which is one of the qualitative research designs, was used in the study. Two 4th grade elementary school students have participated in the study, one with high mathematics achievement and the other with low mathematics achievement. Participants have been selected according to purposive sampling method. Mathematics achievement of the students have been determined through their mathematics scores and the information obtained from their teachers. Data was collected through semi-structured interviews conducted with the students. Descriptive analysis was used in the analysis of the data. As a result of the research it was found that students' mathematics achievement is parallel to their mathematics self-efficacy beliefs. It was found that the student, whose mathematics achievement is high, has positive experiences concerning the sources of self-efficacy, whereas the student, whose mathematics achievement is low, mostly encounters negative experiences.