Imtinan, Nafadila (2023). The Correlation Between Situational Interest in Reading and Vocabulary Size of Sixth Semester English Department Students of Mulawarman University. Thesis. English Department. Faculty of Teacher Training and Education, Mulawarman University. Advisors: (1) Dr. Desy Rusmawaty, M.A. (2) Dr. Phil. Maria T. Ping, M.Sc. The study supported by several previous studies as background to show situational interest in reading had correlation to vocabulary size. This study was conducted to investigate whether there is a significant correlation between situational interest in reading and vocabulary size of the sixth semester students at English Department of Mulawarman University in academic year 2022/2023. The design of this study was quantitative correlational study since the researcher tried to find out the correlation between two variables. The researcher took 50- sixth-semester students of English Department as the sample. The instruments of this study were questionnaire consisting of 12-items to check their level of Situational Interest in Reading and measured by using interval scoring, Vocabulary Size Test consisting of 140 items which for every correct answers multiplied by 1.000 to measure their scores of the vocabulary size, and Pearson product moment formula was used to analyze the correlation between the two variables. First, research findings showed that the mean score of the reading interest was 47 in the range of 40 – 48, that meant the level of students’ situational interest in reading was “Moderate”. Second, it was found that the mean score of the vocabulary size test was 8164 in the range of 3000 - 9000, it meant that the students’ vocabulary size fell in the category of “Mid-Frequency level”. Third, the researcher found that students’ situational interest in reading and vocabulary size had a significant and high correlation with p-value of two variable is 0.00 which means less than significance level (0.05) or 0.00 < 0.05. Also, with N=50 and in the 5% significant level, the r-count is 0.743. It can be concluded that there was high significant correlation between students’ situational interest in reading and vocabulary size of the sixth semester students at English Department 2022/2023. The result showed that students who got high score in their reading interest tend to get high score in the vocabulary size test. In other words, Alternative Hypothesis (Ha) is accepted and Null Hypothesis (Ho) is rejected.