This study aim to examine the effectiveness of plyometric training on muscle physical changes. 24 long jumpers under 12 year old from Cameron Highland were involved in this study. 12 participants were in the control group and another 12 were assigned into treatment group. The training program was conducted twice a week for six weeks. Girth of thigh and calf muscles measurement were conducted during pre-test and post-test. The data were analyzed using descriptive analysis and two-way ANOVA test to determine the mean differences between groups, trial period and interaction of group and trial period. The findings showed that calf and thigh circumference show an improvement for both treatment and control group, in which the treatment group had gained significantly higher achievement. The two-way ANOVA test showed significant differences in enlargement of thigh muscle and calf muscle diameter between groups and trial period. As a conclusion, the plyometric exercises increased circumference of the thigh muscles and calf muscles among long jumpers. The implication of the study is that the proper plyometric training with an appropriate load for primary school athletes need to be implemented to achieve peak performance.