ROVs with computer aided trajectory control and acoustic feedback sensors will be needed for lifting and transportation of objects related to the underwater activities such as inspection, installation and repair of offshore structures. Especially in deep seas, environments are hazardous for human and ROVs become very useful tools. These ROVs are also demanded in the activities related to huge offshore platforms for space utilization. In the repair and inspection of such structures, the amount of works are expected to be so large that without these ROVs the works can not be accomplished. These ROVs are thought to have relatively complicated configurations due to large thruster, and the dynamics will be highly nonlinear and have uncertainties in their dynamics compared to the streamlined vehicles. The nonlinearlities depend on velocity, and hydrodynamic coefficients are poorly known. When such system are controlled, control method must be able to handle the nonlinear dynamics and uncertainties in parameters which depend on the motion of ROV. Gain-scheduled technique which uses a set of linear controllers for different combinations of speeds and directions of motions, can handle these problems. Although theoretically possible, the number of linear controllers become so large that this method is not practical. Another technique which is able to handle the problems and adopted in this paper is sliding mode technique. If the bounds of uncertainties in parameters which determine the dynamics of ROV are known, the technique can incorporate them in the formulation and allows uncertainties in the system. Dynamics change in ROV caused by lifting objects can also be treated as a uncertainty in parameter and control is expected to be robust. The trajectory control of ROVs is examined experimentally by model basin test using a small ROV with four thrusters. Surge and Yaw are controlled by two thrusters in horizontal plane. Heave and Sway are controlled by thrusters arranged diagonally in vertical plane. Ultrasonic positioning system was developed for the basin test. The ROV used in the test was relatively small, and high accuracy of one millimeter order was required for the positioning system and realized by using ultrasound of high frequency.