Background and Purpose: This objective of this study is to find out the effect on the muscle power and functional change before and after the cybex strengthening program to the children with spastic diplegia for the improvement of the muscle power after operation. Subject and Methods: Subject is an inner patient with cerebral palsy in the Severance rehabilitation hospital as of August 27 2007. She got an orthopedic operation in March 13. 2007 and cast removed in May 31. We trained them for the knee extension and flexion concentric contraction training by using Cybex 770 dynamometer 15 times for 4. Results: (l) GMFM test: The result of the GMFM before the training showed 75.9% but that of the GMFM after the training increased 85.4%. First of a11, it showed the biggest change in the E item. (2) TUAG test: The result of TUAG test before the training is 33.1 seconds on average. (3) Strengthening test: The peak torque of concentric extensor to the right side increased 1.5%, total work decreased 2.5%.The peak torque of concentric flexor to the right side increased 0.5% and total work 3.0%, too. The peak torque and total work of concentric extensor to the left side had no change, that of concentric flexor increased 3.0%, 1.0% each. (4) Tardieu Scale: The knee flexor to the right side increased 200 from 400 to 60。’ the left side 350 from 40。to 750 . The knee extensor to the right side increased 50 from 1350 to 140 。’ the Ieft side had no change as 140 。 Conclusions: To find out the functionaI improvement and building up of the muscle power after cybex strengthening program to the children with cerebral palsy who got an operation.