In recent years, underwater robotic vehicles have become an intense area of oceanic researches because of their emerging applications. Modelling and control of underwater vehicles are extremely difficult tasks. Many interacting factors which are involved in underwater vehicle dynamics can cause oscillatory or unstable operations. This thesis is one of the few works deals with modelling and control of a Variable Mass Underwater Vehicle (VMUV) with six degrees of freedom. In this research, the general form of the governing equations that could be accounted as an essential prerequisite for simulation of a body is derived for an underwater varying mass body. In this study, the appropriate forms of the aforementioned equations in the body frame and fixed frame are presented and the simulation results of dynamic modelling are analyzed. The analysis of simulation results validate dynamic modelling. Since, the mass of VMUV changes during the operation, both the mass and its center change with time. Therefore, dynamic equations are written around the center of buoyancy and these are more complicated than the equations of the fixed mass underwater vehicles. On the other hand, the hydrodynamic forces and moments, which could be viewed as the main inputs for simulation process, are generally nonlinear functions of system variables and are calculated with their corresponding coefficients. These coefficients, in turn, depend on different system variables and are calculated through the hydrodynamic analysis. It is difficult to accurately determine hydrodynamic coefficients and the dynamics of underwater vehicles; therefore robust control method are suitable choices for controlling them. This thesis presents a new control approach based on sliding mode theory for variable mass underwater vehicles, which is robust to external disturbance and system uncertainties. This control law includes a multivariable exponential function to eliminate the control signal chattering. Through a theorem, state convergence to the sliding surface and uniform global asymptotic stability of the proposed control system are guaranteed based on Lyapunov stability theory for non-autonomous systems. The proposed approach is compared with a conventional sliding mode controller. Computer simulations are done by considering suitable levels of uncertainty in hydrodynamic coefficiets and a bounded disturbance force and moment. The results of simulating the control system for a 6DOF variable mass AUV (REMUS) confirm the validity and effectiveness of the proposed approach. Key words: Variable mass underwater vehicle, parameter uncertainty, sliding mode control, chattering.