The paper presents a control system design of the ROV operated both as towed and self-propulsive vehicle, which is proposed by Kyushu Univ and named DELTA. First a nonlinear simulator of DELTA is discussed. In order to validate the nonlinear simulator developed, an experimental step response of DELTA is compared with nonlinear and linear simulation results when thruster inputs are increased 20% up. Next, an LQI control system for DELTA is discussed. A linear model of DELTA around an equilibrium point is obtained from the nonlinear simulator, in the form of state space representation with the 10 state variables, 2 input variables and 4 output variables. The possibility is investigated to control the depth and the course by equalized thrusters and differential thrusters respectively. Successful results of the designed control system are shown with simulations by connecting the linear controller with nonlinear simulator of DELTA.