The aim of this study is to examine the implementation of mastery learning method in teaching Islamic religious education at a state senior high school in Sigi regency in Indonesia. This study used a qualitative method in which the data was gathered through field observation, in depth interviews, and written document analysis. This study was conducted in a state senior high school in a regency in Indonesia. The focus of this study is the use of mastery learning methods in teaching the subject of Islamic religious education to the students of the school. Our findings show that the mastery learning method can increase the students achievement in the Islamic religious education subject. Teachers in the school implemented the method using classical approach which involved steps such as teaching preparation, teachers readiness, orientation, apperception, material presentation, and evaluation. Our study also shows that the students were well-motivated in studying the subject. The motivation was increased as the result of the freedom given to the students during learning process.