Purpose : This study is to investigate difference in the isometric muscular function of knee joints according to the time of participation in rehabilitation exercise in patients who had anterior cruciate ligament reconstruction. Methods : The subjects of this study were patients by sports injury or accident in the sports rehabilitation center of G hospital. The early exercise program group (n=7) started functional ability exercise from 2 weeks after the surgery and the late exercise program group (n=7) from 6 weeks after the surgery. Statistical analysis was used repeated measure ANOVA to test mean difference by using SPSS 18.0 for windows. Results : First, as to quadriceps femoris muscle according to the time of participation in exercise program, significant difference was observed according to interaction and time. Second, as to hamstring muscle according to the time of participation in exercise program, significant difference was observed in muscle strength according to time. Conclusion : This results suggest that if the effect of exercise program is similar between the early starting group and the late starting group, it is not necessary to have a long period of fixation as in the late exercise program group but is desirable to start functional ability exercise early in order to relieve pains in the knee joints.