The purpose of this study was to describe the alteration of stretch reflex and electrical activity of lower leg muscle in cerebral palsy by the water temperature. We used ten cerebral palsy. Participants classified according to each group in $29^{circ}C$, $32^{circ}C$, $35^{circ}C$ by the water temperature. All subjects participated 3 session, which at least 1 week between session. The test was measured continuously pre test, post-test by thermometer and surface electromyography(sEMG). The following results were obtained; 1. Superficial temperature were significantly decreased in $29^{circ}C$(p<0.001) group but were significantly increased $32^{circ}C$, $35^{circ}C$ group(p<0.001). 2. Muscle stretch reflex were significantly decreased in $32^{circ}C$(p<0.05) and $35^{circ}C$(p<0.001) group. 3. Stretch-evoked EMG were significantly increased in $29^{circ}C$ group(p<0.01). These results lead us to the conclusion that changes of stretch reflex and electrical activity of lower leg muscle in cerebral palsy were influenced by the water temperature. Therefore, a further direction of this study will be to provide more evidence that a moderate water temperature have an effect on muscle tonus in a patient with cerebral palsy.