This research was conducted to investigate the effectiveness of twenty question game in increasing student’s mastery in yes/no questions. It was done especially for the tenth grade of SMKN 1 Banyumas. It consisted of 68 students. Two classes were selected as an experimental group and control group and both classes got different treatment. Each group consisted of 34 students. The use of twenty question game as a technique of treatment was implemented in the experimental group, while in the control group the students were taught as usual without giving the treatment. The data were gained from tests (pre-test and post-test), which were conducted to the experimental and control group. In the pre-test, the mean of experimental group before getting treatment using twenty question game was 49.85, while the control group’s mean was 53.53. In the post test, the mean of experimental group after getting treatment was 80.88 while the mean of control class was 73.97. It can be seen that the students’ grammar score in experimental group was significantly higher than the control class.Then, it was also supported by t-test computation result. The t-obtained was 1.712 t-value at the level of significance 5% for N 68 which has value1.668and the degree of freedom 66. Thus, it could be concluded that the hypothesis was accepted, since t-obtained was higher than t-table (1.712 >1.668).