The aims of this research were to find out: the correlation between students‟ vocabulary mastery and their essay writing skill at Muhammadiyah University of Purwokerto. This research was co-relational in nature and the participants were 34 students or 25 percent of the population (137). The technique which was used to determine the participants was random sampling technique by means of lottery. The data was taken through two kinds of test namely Vocabulary Size Test which was developed by Batia Laufer & Paul Nation and essay writing test. The analysis showed that the result of product moment correlation computation between vocabulary mastery and essay writing skill was 0.434. It was higher than the value of r-table which was 0.339 in the level of significance 95%. Therefore this indicated that there was significant correlation between vocabulary mastery and essay writing skill.