The aim of this research was to find the out the students‟s ability in vocabulary size at sixth grades of Muhammadiyah University of Purwokerto in academic 2016/2017. The method of this reseach was descriptive statistic. The population were all of the students in English Education Department in Muhammadiyah University of Purwokerto in sixth grades. The data collected used vocabulary test. Which were analyzed validity and reability. The computation of the data to find out the the students‟s ability in vocabulary size used. Based on the result of the reseach, the reseacher was found that the results of research showed that the level of ability in the vocabulary size of the students in bad category were 9 respondents from 60 respondent, fair category 17 respondents from 60 respondent, good category 25 respondents from 60 respondent, and in very good category as many as 15 respondents from 60 respondent. This shows that the ability in vocabulary size of students in English Education Department in Muhammadiyah University of Purwokerto at a good level of ability.