Remotely operated vehicles (ROVs) and underwater robots have been greatly involved in scientific related research and commercial utilization of underwater resources for the past few decades. They are operated at scenes where conditions are beyond the physical limitation of human divers or are too hazardous for them. -- To gain experience with the design, fabrication, control and operation of subsea robot, an hydraulic subsea robot is being developed at Faculty of Engineering and Applied Science of Memorial University of Newfoundland. This uses a plunger/waterjet device to control depth. When operating with the plunger, variation of displaced volume generates a positive or a negative buoyancy and thus controls the robot's vertical movement and depth. When operating with water- jet, movement is controlled by the combination of the buoyancy and the thrust generated from the waterjet squeeze. -- This thesis explains in detail the development of the control system of the robot, from simulation to test in a water tank, and reviews the systems stability, time response and frequency response performance.