The purpose of this study was to find out the correlation between Grammar Mastery and Reading Comprehension of the fifth semester students’s of English Department of Mulawarman University.This study was a quantitative study. The samples of the study were 89 students of the fifth semester. The answers of the problems of the study were analyzed by using Pearson product-moment analysis in SPSS 25 Program. The instruments of the study were Grammar Mastery Tests and Reading Comprehension Tests. This study revealed the following finding: it was found that the null hypothesis (Ho) was rejected and alternative hypothesis (Ha) was accepted. It can be seen from the data analysis that rxy = 0.902. As a result, there was correlation between the students’ grammar mastery and their reading comprehension and the correlation was strong. Then, it can be concluded that if the students have high scores in grammar, they will also have high scores in reading comprehension, and the other way around.