The purpose of this study was to describe the alteration of muscle action potential of spastic upper extremity in a patient with cerebral palsy by the water temperature. We used seven patients with cerebral palsy. Participants classified according to each group in $29^{circ}C,;35^{circ}C$ by the water temperature. All subjects participated 2 session, which at least 1 week between session. The test was measured continuously pre test, post-test by thermometer and surface EMG. The following results were obtained; 1. In changes of surface temperature, wrist flexor and extensor were significantly decreased in $29^{circ}C$ (p<0.001) group but were significantly increased $35^{circ}C$ group(p<0.01). 2. In changes of surface temperature, wrist flexor and extensor were significantly differenced between $29^{circ}C$ and $35^{circ}C$ group(p<0.001). 3. In changes of muscle action potential, wrist extension antagonist were significantly increased in $29^{circ}C$ group(p<0.05). 4. In changes of muscle action potential, wrist flexion agonist were more significantly increased in $29^{circ}C$ group(p<0.01). 5. In changes of muscle action potential, wrist extension antagonist were significantly differenced between $29^{circ}C$ and $35^{circ}C$ group(p<0.05). These results lead us to the conclusion that changes of muscle action potential of spastic upper extremity in a patient with 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 tone in a patient with cerebral palsy.