This research was conducted to investigate the effectiveness of Change Chairs Game in increasing student’s mastery in grammar especially future conditional sentence. It was done especially for the ninth grade of SMP N 3 Purwokerto. It consisted of 60 students. Two classes were selected as an experimental and control group and both classes got different treatment. Each group consisted of 30 students. The use of Change Chairs Game as a technique of treatment was implemented in the experimental group, while in the control group the treatment was conducted in a conventional way.
The data were gained from tests (pre-test and post-test), which were administered to the experimental and control group. In the pre-test, the mean of experimental group before getting treatment using Change Chairs Game was 60.4 (fair), while the control group’s mean was 63.06 (good). In the post test, the mean of experimental group after getting treatment was 85.86 (very good) while the mean of control class was 78.53 (good). 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 was16.798with t-value at the level of significance 5% for N 60 which has score 2.042 and the degree of freedom 58. Thus, it could be concluded that the hypothesis was accepted, since t-obtained was higher than t-table (16.798>2.042).