Despite vocabulary being essential, there still student have difficulties mastering vocabulary. This study aimed to investigate whether using a guessing game can improve students' vocabulary mastery for second-grade students at SMP Islam YPUI Banda Aceh. The research method was quantitative research. Specifically, a pre-experimental research design was carried out in a single class. The data was obtained using a test (pre-test and post-test). The study's samples were chosen using a random sampling technique with 26 students. The result showed that their post-test scores were higher than their pre-test scores. That was, the mean of the post-test was 81,30, while the mean of the pre-test was 64,57. It means an increase in value after being given treatment using a guessing game. As a result, there was a difference between students' test results in the Pre-test and Post-test. Furthermore, most of the students' responses towards implementing the guessing game in improving students' vocabulary were positive.