This research was aimed at measuring the level of the students‟ mastery of relative clause. This research used descriptive method. For this purpose, the sixth semester students of English Department of Muhammadiyah University of Purwokerto in Academic year 2016/2017, which consisted of ninety five students and forty eight of them (50%), were taken as the sample. The writer used test to collect the data, the result of which was then analyzed. Based on the analysis, it was revealed that the student‟s mastery in relative clause of English Department was in good category since the mean was 71.01%.
The percentage of students‟ mastery who belong in very good category were 10 students or 20.83%, good category were 23 students or 47.91%, fair category were 13 students or 27.08%, and the last was bad category since there were 2 students or 4.16%.