In learning a language, there are four skills to explore namely listening, speaking, reading, and writing. To master all of them, vocabulary becomes one of the essential aspects to explore. Students are supposed to have many vocabularies in order to perform well in English language learning. There are two variables used in this research; vocabulary mastery as dependent variable and ‘Big Describer’ online game as independent variable. This study aims to find out if ‘Big Describer’ online game is effective to improve students’ vocabulary mastery. The sample of the research is 50 tenth grader students of MA Asy-Syarifah in the academic year of 2021/2022. They are from 2 different classes, X Language-01 as experimental group and X Language-02 as control group. So there are 25 students from each group participating. The study uses quasi-experimental designs with test as the data collection instrument. There are 2 kinds of tests done, consisting of pre-test at the beginning of the research and post-test at the end of the research. These two tests contain similar test items, test rules, and allocation time. Between the tests, students in each class get 3 meetings of treatment with ‘Big Describer’ online game for the experimental class, and conventional teaching strategy for the control class. The results of the tests done show the mean score of students in experimental group improves 32.4 points from 55 to 87.4. While the mean score of students in control group improves less than 2 points from 55.4 to 57. Based on the paired samples T-test done for each group, the sig.(2-tailed) of experimental group is 0.000. Since it is less than 0.05, there is a significant difference between students’ vocabulary mastery before and after the treatment. In opposite, the sig.(2-tailed) of the control group is 0.018 which is more than 0.05, so it is insignificantly improved. Finally it can be concluded that ‘Big Describer’ online game is effective to improve students’ vocabulary mastery.