The increasing cost of electricity often drives the famers of the countries of the southern shore of the Mediterranean, to adopt the natural ventilation in order to provide greenhouse aeration. The roof and sidewall vents are opened to allow the excess heat to escape and cooler outside air to enter during daytime. During night time, these openings are used mainly to regulate the excess humidity in greenhouse which causes damage on plants due to the development of Botrytis cinerea. This paper presents a computational fluid dynamic (CFD) comparative study of the effect of these roof and sidewall ventilation openings on airflow circulation and diurnal and nocturnal greenhouse climate distribution to assess their effect. The investigation was conducted in a one hectare canarian-type greenhouse, the most widely used in Morocco, with a mature tomato crop. The simulations were performed with the CFD model based on solving partial differential equations, which represent conservation laws for the mass, momentum, and energy, using CFD finite volume method (FVM). This CFD model takes into account the virtual crop as a porous medium using the Darcy-Forchheimer model restricted to its inertial terms. Simulation results show that opening configurations strongly affects the airflow circulation under the studied greenhouse, which can generate a heterogeneous climate at the canopy level, especially during daytime. Results have illustrated also that there is a reverse flow from the leeward end to windward end part of the greenhouse at the crop level. Closing the north-south sidewall ventilation openings contributes significantly to the inside air velocity increase which can decrease the diurnal air temperature at the crop level. Conversely, during night-time, climate distribution at the crop level is homogeneous on the whole greenhouse.