In this research the effect of mastery learning model on academic achievement of students is investigated. Whether the effect of mastery learning on academic achievement changes according to the field and education level or not is also investigated. This research is a meta-analysis study and consists of 24 research (dissertations, articles and oral presentations). According to the results of the study, all of the studies have positive (+) effect size. It was found that 50% of the studies that were included to the meta-analysis had huge effect size, 29,17% of them showed large, 12,5% of them showed middle and 8,33% of them showed very large effect size. According to random effect model, average effect size of the studies that were included to this research is calculated as 1,546. According to this finding, mastery learning approach has a very high and positive effect on academic achievement of students. While the academic achievement of the students doesn't change in terms of the fields that the lessons applied; a significant difference is found in terms of education level. Mastery learning approach has a huge effect on academic achievement for primary school and high school level.