The purpose of this study was to investigate the effect of watsu therapy on muscle tone in adult hemiparesis patient after stroke. The subject was a 66 year old patient with left hemiparesis whose self gait was independent with minimal assist. The subject was ambulatory with some degree of limitation and demonstrated a little tremor and asymmetry in the body. A multiple treatment design(A-B-C-B) for a single-subject research was used for this study. The watsu therapy consisted of basic moves, head cradle, underfar leg, near leg cradle in watsu I (transition flow), for 25 minutes, which lasted 12 sessions for a total 21 sessions. A baseline was conducted for a first time and a general ROM exercise was conducted after the first watsu therapy. Tone assessment scale was tested : 1. posturing at rest 2. response to passive movement 3. associated reaction Comparing to the baseline and general ROM exercise, The results showed that the watsu therapy had some positive effects on improvement of three tests and there was a more positive effects of both response to passive movement and associated reaction.