The objectives of the study were to find out the correlation between students’ grammar mastery and writing ability and to find out what aspect of writing has the most correlation on students’ grammar mastery. This research used quantitative approach. The subject of the research was the first grade students of MAN 1 Bandar Lampung. It consisted of thirteen classes; the total number of population was 439 students. By using sample random sampling, the sample of this research was 40 students taken from twelve classes (3 and 4 students in each class). A set of the grammar test in the form of multiple choices was used to measure students’ grammar mastery and writing test was used to measure students’ writing ability. Pearson Product Moment Correlation in SPSS 25.0 for windows was applied in this research to analyze the data. The result indicated that there was a correlation between students’ grammar mastery and their writing ability since the significant value was 0.730 resided between 0.600 - 0.800, which means the strength is high correlation. The data indicated that Sig. (2- tailed) = 0.00 which was lower than 0.05. This suggests that a student who has high scores in grammar mastery, she or he also get a good score in writing. It was assumed that language use was the aspect of writing has the most correlation on students’ grammar mastery because; it was found that during the writing test, the students showed various errors, such as using incorrect tenses and unstructured sentences. From the research findings, it can be concluded that there was a positive correlation between students’ grammar mastery and writing ability in descriptive text and language use was the aspect of writing that most correlated with grammar mastery.