The objective of this study was to realize the butterfly stroke by the swimming humanoid robot “SWUMANOID”, and to investigate the swimming performance of the robot. The robot was modified from its original condition in order to reduce the weight of the lower body. The swimming motion of the robot was generated based on that of an actual swimmer. By preliminary simulations, it was found that the swimming speed became significantly low when the stroke cycle was 2.0 s. In the experiment, successful propulsion was confirmed for the stroke cycles of 3.45 s. The swimming speed was 0.181 m/s.