The effect of strengthening exercise in a cerebral palsy with lower extremities tendon lengthening was evaluated. He did isotonic exercise with progressive resistance on knee extensor, knee flexor and ankle plantar flexor. The plantar pressure of both foot during stance phase and the differences of plantar pressure between the left foot and the right foot were measured by foot scan before and after the exercise. And the muscle power of bilateral lower legs was measured by cybex. Finally the change of the gross motor function was evaluated by GMFM. The Results were following : 1. There was a significant in the symmetry of plantar pressure of foot in lower extremities after strengthening exercise for muscle power. 2. It was increased muscle power of knee extensor flexor in all joint angles ankle plantar flexor but decreased difference of muscle power in lower extremities. 3. It was increased functional ability of gross motor which crawling & kneeling, is standing, walking, running, jumping. Strengthening exercise for muscle power in cerebral palsy child is helpful for improving the muscle power, functional ability of gross motor. But the whole body considered treatment is more effective than one muscle activity for symmetry gait of normal pattern.