A dictionary can be a useful tool for foreign language learners in mastering vocabulary. Learners who had enough vocabulary can express themselves and they will not get stuck in reading, writing, listening and speaking. This research aims to know (1) the use of English dictionary of the students of EED of UMY batch 2013, (2) vocabulary mastery of the students of EED of UMY batch 2013, and (3) the correlation between the use of English dictionary and vocabulary mastery of the students of EED of UMY batch 2013. This research used a decriptive quantitative research to find out the correlation between both variables. The sample of this research was 66 students from the population of 194 students batch 2013. The researcher used questionnaire and vocabulary test as the instruments. The data analysis using Microsoft Excel and SPSS program indicated that the findings that the use of dictionary of the students of EED of UMY batch 2013 was in very high categories with the mean value 4.06. Moreover, this research found that vocabulary mastery of the students of EED of UMY batch 2013 are in excellent categories. However, the data analysis also reveals that there is strong correlation, probability value was 0.641. The number of significant correlation was 0.000<0.05, which means that there is a significant correlation. Thus, Ha (alternative hypothesis) is accepted and Ho (null hypothesis) is rejected. The use of English dictionary can improve students’ vocabulary mastery, because there is a significant correlation between the use of English dictionary and vocabulary mastery of the students of EED of UMY batch 2013.