The purpose of this study was to investigate the effect of peer tutoring on mathematical inclination and mathematical communication ability of peer tutor. For the purpose of this study, research questions were established as follows: 1. How does peer tutoring affect to the mathematical inclination of peer tutors? 2. How does peer tutoring affect to the mathematical communication ability of peer tutors? To answer the research questions, four 5th grade peer tutors were selected for qualitative case study in an elementary school located in Goyang-si, Gyeonggi-do. Before and after 11 weeks of peer tutoring in their mathematics classes, mathematical inclination, mathematical communication ability of peer tutors were examined. For qualitative analysis, peer tutors were asked to complete worksheets, self-evaluation, journal for their peer tutoring in daily basis during the experiment. By comparing the scores in mathematical inclination test and mathematical communication test before and after the treatment and analyzing the data gathered for qualitative analysis, the conclusions were drawn as follows: First, Peer tutoring has positive effects on the mathematical inclination of peer tutors. Scores for mathematical inclination of peer tutors after the treatment increased and qualitative analysis showed positive change in their attitude toward mathematics. Second, Peer tutoring has positive effects on the mathematical communication ability of peer tutors. Scores in the performance assessment for mathematical communication ability of peer tutors after the treatment increased. Also qualitative analysis showed that peer tutors tried to develop various ways to solve a problem and explained them to their peer tutee sophisticatedly.