Vocabulary is one of the most important language aspects in learning English. Through mastering sufficient vocabulary, English students will find easiness to speak, listen, read and write English. This research was conducted to find out the problems faced by students and the strategies used in improving vocabulary mastery. The subjects of this study were students of class XI SMK Muhammadiyah 1 Purbalingga. This research used descriptive qualitative and quantitative research method. The sampling used proportional random sampling technique. Respondents from this research amounted to 53 out of 5 classes, there are TKJ 1, TKJ 2, TKJ 3 and TKR 1, TKR 2. The instrument used was a questionnaire. The results in the form of percentage of the questionnaire showed that the vocabulary mastery of the respondents was categorized negative. The score for the assessment of the answer choices chosen by the students when filling out the questionnaire was a score of 1. It means that the students' vocabulary mastery is very poor. Problems in improving vocabulary mastery include intrinsic factors (pronunciation, spelling, length and complexity, grammar, meaning and range, connotation and idiomaticity) and extrinsic factors (family, environment and economy). The strategies the students used were metacognitive strategies, memory strategies, cognitive strategies, compensation strategies, affective strategies and social strategies.