The objective of this research was to investigate the improvement of students’ medical vocabulary mastery through seminars in English. The design used in this research was classroom action research that includes planning, action, observation, and reflection. The result of the study showed that Sig. is 0.000 <
0.005, so there is a statistically significant difference in the average value of test scores before and after students participating in seminars in English. It shows that there was an improvement of the students’ vocabulary mastery before and after the research.