ABSTRACT Batubara, S. F. 2021. The Correlation Between Vocabulary Mastery and Reading Comprehension toward Writing Ability of The Fifth Semester Students of Mulawarman University Samarinda. Thesis English Department. Faculty of Teacher Training and Education Mulawarman University. Samarinda. Advisors: (1) Dr. Aridah, M. A. and (2) Satyawati Surya, M. Pd. One of difficulties in writing for learners is if they do not have stocks of vocabulary. So that, vocabulary mastery has a significant role in writing ability. Besides the factor of students‘ vocabulary mastery which can affect the writing ability, the research is also interested to see another factor, that is, reading comprehension. In this research, the researcher would like to see how high the correlation between vocabulary mastery and reading comprehension to their writing ability. By knowing this, learners will be able to find out which one gives more contribution when they want to write well. This research was aimed to find out the students‘ vocabulary mastery, reading comprehension, writing ability, and correlation between students‘ vocabulary mastery and reading comprehension toward writing ability of the fifth semester students of Mulawarman University Samarinda. The design of this study was quantitative method with correlation design. The sample of this study was 50 students of the fifth. The instruments of this study were vocabulary test, reading test and writing test. In analyzing the data, the researcher first scoring the vocabulary test, reading comprehension test, and writing test, correlated the score of students‘ vocabulary mastery and reading comprehension to their writing ability using Product Moment Formula and then tested the hypotheses based on the result of coefficient correlation between students‘ vocabulary mastery (X1) and their reading comprehension (X2) toward writing ability (Y) by Multiple Correlation Regression. The findings of this study showed that, the score of coefficient correlation between reading comrpehension (X2) and writing ability (Y) was 0.686 and the score of coefficient correlation between vocabulary mastery (X1) and writing ability (Y) was 0.425. It meant that the coefficient correlation between X2 and Y was bigger than coefficient correlation between X1 and Y. Therefore students‘ reading comprehension influenced writing ability more than vocabulary mastery. Keywords: Correlational Study, Vocabulary Mastery, Reading Comprehension, Writing Ability