:Active control of water behavior is inevitable in the daily life using water, such as hand/face washing and bathing, under the micro-gravity environment in space. Water used for hand-wash and body cleaning clings to the body skin by the surface tension effect, and shed water droplets float around in a washing chamber or in a crew cabin. Water behavior during hand-washing activity in a free air stream was investigated on the NASA KC-135 jet plane introducing 15-20 seconds of micro-gravity condition at parabolic flights. Downward air stream was proved to provide quasi-gravity force to wash down water droplets to the bottom of the chamber.Then the air flow analysis was conducted to predict water droplet behavior in a chamber designed for space hand washer. Disturbance of air flow by the hands inserted from an opening to the chamber was analyzed. By-passing a opening of air flow in the chamber to the hand insertion opening will prevent the water droplets from escaping from washing chamber into the crew cabin. The status of on going study to establish design technique to prevent water splash by the controlled air flow is presented.