Purpose: We investigated to evaluate the effectiveness of water-based exercise (WE) program on respiratory functions for children with spastic diplegic cerebral palsy (CP). Methods: Fourteen children with spastic diplegic CP were randomly assigned, to either the experimental group (EG, n=7), or the control group (CG, n=7). Respiratory function was measured by a spirometer, a CardioTouch 3000S ( Bionet, Seoul, Korea) at a chair-sitting posture. Forced vital capacity (FVC), forced expiratory volume at one second (FEV1), peak expiratory flow (PEF) were measured. The intervention program will last 8 weeks, with three 40 minutes sessions per week (24 training session). The usual care and the addition of a WE program, were compared in the CG and EG, respectively. Results: The EG showed a significant increase in the FVC, FEV1, PEF after training (p<0.05), whereas there was no significant difference in the CP after training. In the EG, FVC increased significantly, compared to the control group (p<0.05), but not FEV and PEF. Conclusion: These findings suggest that WE program have an effect on the respiratory function in children with spastic diplegic CP.