The variable ballast is used to control the buoyancy of the body in Underwater Robotic Vehicle (URV) system. The variable ballast can also be used as a motion actuator in vertical plane. Many variable ballast mechanisms have been developed by researchers. In this paper, development of a variable ballast mechanism is presented. This variable ballast mechanism is designed to make the water always fulfill space in the variable ballast tank with the varying volume. This variable ballast is utilized for motion actuator in vertical plane of a spherical URV. By using physical laws, a nonlinear dynamic model of the URV and variable ballast is derived. The model is simulated using Simulink in MatLab. © 2008 IEEE.