Numerical study of double-diffusive natural convection flow and entropy generation in 3D trapezoidal solar distiller was performed using computational fluid dynamics (CFD). In this research the flow, provoked by the interaction of chemical species diffusions and the thermal energy, is assumed to be laminar. Using potential vector-vorticity formulation in its three-dimensional form, the governing equations are formulated and solved by the numerical methodology based on the finite volume method. The main objective is to analyze the effects of buoyancy ratio for opposed temperature and concentration gradients and to focus the attention on three-dimensional aspects and generated entropy. The occurring heat and mass transfer are depicted by the dynamic and temperature fields of the flow inside the trapezoidal cavity. It has been found that the flow structure and the heat and mass transfer are sensitive to the value of buoyancy ratio.