The aim of this study was to determine the relationship if any, between Game-Based learning and mathematics achievement of grade seven students in a middle secondary school in Bhutan. The research using quasi-experimental design, was carried out in 2015 in one of the middle secondary schools in Bhutan over a two-month period.The participants consisted of 70 seventh graders in two classes. One class was assigned to be the experimental group and the other the control group. The experimental group learned using game-based learning approach, while the control group learned using a traditional teaching approach. The data obtained in the study were analyzed by the computer program SPSS 21.0. Arithmetic means and standard deviations were calculated for each group. The result showed that the learning achievement of the students in the experimental group was significantly better than that of the students in the control group. The mean difference for the experimental group over the two month period was 9.29 which is significantly higher than that of control group which was 3.40. These data confirm that there is a strong relation between game-based learning approach and mathematics achievement. With respect to attitudes toward mathematics there was a similar result over the two-month period. Most of the students revealed quite positive attitudes toward the use of the game-based learning approach. The mean in the pre-test of the experimental group was 2.06 and 4.61 in the post-test. The mean score had increased by 2.55 which indicated that game-based learning had a positive effect on student attitudes towards mathematics.These results should encourage school leaders in Bhutan and elsewhere to consider adopting Game-Based learning approach for the teaching of mathematics for middle school age students.