This study aimed to determine the effect of mastery learning model supported with reflective thinking activities on the fifth grade medical students' academic achievement. Mixed methods approach was applied in two samples (n = 64 and n = 6). Quantitative part of the study was based on a pre-test-post-test control group design with an experiment group (32 students) and a control group (32 students) selected among the fifth grade students starting their internship training in Infectious Diseases and Clinical Microbiology Department. Afterwards the qualitative data were collected through semi-structured interviews with six students, selected from the experimental group and the faculty member conducting the study. Findings from the quantitative part indicated that there was a significant difference between post-tests scores of the experiment and control groups favoring the experiment group [t(62)= -2,815; p=0.007]. Quantitative results revealed positive attitudes toward the applied model in terms of increasing achievement.