Different theoretical and experimental studies carried out in the field of solar distillation with green-house effect, have shown that global efficiency of a simple solar still are affected by physical and building parameters, especially by the difference of temperature between the evaporation surface and that of the condensation. Optimising this difference allows us to obtain a solar still with a better efficiency. A mathematical modelling has been carried out aided by some basic and simplified hypotheses, according to overall thermal balances and appropriate heat and mass coefficients, while taking into consideration a stagnant area in the solar still. Resolution of equations, based on method of finite differences, has shown that a better efficiency is obtained at a maximum temperature difference, as well as we can obtain this latter by a low glass thickness, a gradient (angle of inclination) closer to that of the area latitude, in which our solar still is placed, a low thickness of the solution to be distilled and a high wind velocity. Results issued from this study show clearly the importance of a cooled condensation surface and a hotter evaporation surface.