Animals have behavioral characteristics, which is called "taxis". These are the behavior to go toward or away from some kinds of stimulus such as light, water flow, chemical substances, and so on. In this paper, the authors focused on one kind of taxes called phototaxis, and attempted to apply it as the motion control system of a fish type robot. To mimic the taxis, two photo sensors are installed in the newly developed robot body. In past work, authors had a problem with its battery power of the robot. To resolve this, a lithium-ion battery is employed due to its high capacity and high rate of discharge. In this paper the authors introduce an improvement to the robot. © 2010 by The International Society of Offshore and Polar Engineers (ISOPE).