The main purpose of this study was to get empirical data about Vocabulary knowledge and students‟ writing skill and to analyze the correlation between vocabulary knowledge and students‟ writing skill. The subjects of this research were the sixth semester students of English Education Department of Universitas Muhammadiyah Purwokerto in 2017/2018 Academic Year. The number of students were 30. The type of this research was a survey research. Data collection was done by giving two tests: English Language Proficiency Assessment (ELPA) medium level vocabulary test and writing test. Pearson product moment formula was used to measure the correlation coefficient. The findings reveal that firstly, the vocabulary knowledge of the students was on average 3000 – 3500 words and their writing skill was at fair category. Secondly, based on the data analysis used product moment correlation, the result of calculating data showed that the correlation coefficient between students‟ vocabulary knowledge and their writing skill was 0.095. Referring to the r table of the level significant 5% for the number of tested (N) = 30, it showed that r score obtained was lower than r table (0.095 ≤ 0.361). Therefore, there is no significant relationship between vocabulary knowledge and their writing skill.