The students study vocabulary through dictionary in order that the students become mastery of language. Students can read a dictionary everywhere. Based on the writer’s observation, there are many students who usually open a dictionary and it makes their vocabulary mastery good. In the write’s personal experience, the researcher usually reads a dictionary via mobilephone and dictionary book to search and know easy and difficult English words so that the researcher can speak English. Based on the background, it has been emphasized by the researcher’s experience that the use of dictionary is important in learning a language. This research was intended to investigate the frequency of ELED UMY students’ use of dictionary, to examine the students’ vocabulary mastery of ELED UMY, and to find out the correlation between the use of dictionary and students’ vocabulary mastery. Research design used quantitative method. Research setting was PBI UMY. Population was students of ELED UMY batch 2014 with 31 students as the sample. The findings showed that the use of vocabulary of ELED students is at the frequent level. The students’ vocabulary mastery is at the good level. The last finding showed that there is a negative correlation between the use of dictionary and the vocabulary mastery among ELED students.