Natural ventilation contributes to the improvement of thermal comfort and quality conditions of the internal air and at the same time it is recognized very effective technique when applied properly. The present project refers to the numerical predictions of air velocities in cross and single-sided naturally ventilated rooms which are considered along with atmospheric zone. This is done for different sizes of the opening at particular height of the wall. Continuity and Momentum equations have been solved by control volume method. SIMPLE and SIMPLEC algorithm are used to solve these equations. Steady, k-ε turbulence model and incompressible flow of a constant property fluid under Boussinesq’s approximation have been considered. Air velocities were taken at various heights from middle of the opening and compared for single side ventilated and cross ventilated rooms. Air flow rate increases with increase of width of the opening.