A theoretical investigation on the performance prediction of solar still is presented in this paper. A solar still of conventional type is considered. The mathematical model is based on time-averaged Navier-Stokes equations. The effect of the variable fluid properties is taken into consideration by using a mixture of air and vapour in the still. A steady state two-dimensional approach with constant temperature boundaries is considered. A discretization schema with finite-difference technique is adapted. The SIMPLER (Semi-Implicit Method for Pressure-Linked Equations Revised) methodology is used. The grid size independence solution is checked for convergence. The numerical results show clearly different zones of circulation with reverse velocity on the inside still glass cover. The numerical results prove the necessity to undertake a numerical investigation before the sizing of solar still. The objective of the present investigation is to present a mathematical model in order to improve the still design, also the obtained results prove to be a good tool for performance prediction according to a given geometry.