This study aims to determine the correlation between formal reasoning ability and physics concepts mastery of Senior High School Nasrani 1 Medan Students in 2018/2019 Academic Year. The method of correlational research was carried out in this study with a sample size of 40 students from a total of 94 students whose withdrawals were carried out by purposive sampling method. The research instrument used was the Formal Reasoning Ability Test which was adapted from the Test of Logical Thinking (TOLT) and the Physics Concept Mastery Test. The results of the analysis of formal reasoning abilities show that the transitional reasoning rate is 12.5%, the initial formal is 37% and the formal is 50%. Whereas, the physics concept mastery test was classified as good, with the highest score being 9.5 and the lowest being 6.5 overall. Students can achieve the minimum completeness criteria (KKM) of 6.5. The magnitude of the correlation coefficient index is 0.756 which is classified as high and the coefficient of determination is 57.15%. The tcount is 7.113 and the significance (pvalue) is less than 0.05, it means that the hypothesis is accepted which states that there is a correlation positive and significant between formal reasoning ability and physics concept mastery of senior high school Nasrani 1 Medan students in 2018/2019 academic year

Keywords : Formal Reasoning Ability, Physics Concepts Mastery