In this study, the investigation of the steady state laminar natural convection heat transfer inside a cavity under different heat fluxes (q ''), different opening ratios (OR), different tilt angles (TA), and different cavity aspect ratios (AR), for top opening position, was performed using Computational Fluid Dynamics (CFD). One side of the cavity is subjected to a uniform heat flux and all other sides of the cavity are assumed to be perfectly insulated. The equations of conservation of mass, momentum and energy were solved using adequate boundary conditions. According to the results obtained, average Nusselt number (Nu(H,avg.)) increases and the average wall temperature decreases, with increase in opening ratio and decrease in tilt angle.