The purpose of the present study was to the investigate the effect of aging in men and women on muscle strength of knee extensor and flexors by using the cybex 6000 isokinetic dynamometer. A total of 100 volunteers participated in this study and were divided into five groups according to their chronological age as follows: 20s, 30s, 40s, 50s, 60s, 10 men and 10 women in each decade respectively. Isokinetic ($60^{circ}{cdot}s-l$) knee extensor and flexor peak torque, peak torque to body weight ratio, opposing muscles(flexor/extensor) peak torque ratio, deficit of peak torque between dominant and non-dominant were measured. The results obtained were as follows: 1) In men, While the aged increased. the peak torque of the knee flexor and extensor statistically sig nificant decreased in the dominant and non-dominant side. 2) In women, Statistically significant difference of knee extensor peak torque was found as the aged increased in the dominant and non-dominant side, but significant difference of knee flexor peak torque did not that. 3) In men, No significant difference in the peak torque of knee flexor to body weight ratio was found as the aged increased in the non-dominant side, but statistically significant difference in the peak torque of knee flexor to body weight weight ratio was found as the aged increased in the dominant side. 4) In women. No significant difference in the peak torque of knee flexor to body weight ratio was found as the aged increased in the dominant side, but statistically significant difference in the peak torque of knee flexor to body weight ratio was found as the aged increased in the non-dominant side. 5) In men and women, While the aged increased, statistically significant difference was found the dominant and non-dominant side in the peak torque of knee extensor to body weight ratio. 6) Peak torque of hamstring to quadriceps ratio of dominant and non-dominant side in men and women were not significantly different as the age increases. 7) Mean deficit of peak torque between dominant and non-dominant side in men and women were not significantly different as the age increases. From these results we conclude a proper exercise program is need before 50s decade to preserve in muscle strength of knee flexors and extensors.