Speaking was one of an important skills in language learning. However, it was not an easy matter to be mastered by students because there were some factors that influence the speaking ability, and one of those factors was vocabulary size. Therefore, this research was interested to investigate the correlation between students’ vocabulary size and speaking ability. The aims were to answer what the students’ vocabulary size was, how the students’ speaking ability was and what the correlation between students’ vocabulary size and speaking ability was. The researcher used quantitative approach with correlational quantitative design to find out the correlation between two variables. The setting of the research was in ELED batch 2016 and it was conducted on February to April, 2018. The data were collected from 85 participants in ELED batch 2016. The researcher used Vocabulary Size Test and the document of speaking ability test score to gain the data of vocabulary size and speaking ability. Then, the researcher used descriptive statistic to identify students’ vocabulary size and speaking ability, and used inferential statistic to reveal the correlation between vocabulary size and speaking ability. The findings showed that the average of students’ vocabulary size was 5.191 word families. It means that students were able to communicate in some topics even with native speakers, write English text, understand some discussions, and understand if someone talks in English. Meanwhile related to speaking ability, the mean score was 57.86 that it was categorized in good level. Furthermore, the finding of correlation showed that the significance value was 0.408. It means that the significance value was higher than 0.05 (0.408≤0.05). Thus, H0 was accepted xiii or there was no correlation between students’ vocabulary size and speaking ability