Studies have been done on the aspect ratio effect on natural convection turbulence using standard k-ε model but further studies showed that k-ω SST model performed better than both k-ε and k-ω model in the whole enclosure. Thus, there was need to do a numerical study on the natural convection fluid flow in a rectangular enclosure full of air using SST k-ω model. The left vertical wall of the enclosure was maintained at a steady high temperature Th of 323K while the right wall at a steady cool temperature Tc of 303K with the remaining walls adiabatic. Time-averaged energy, momentum and continuity equations with the two equation SST k-omega turbulence model were used to generate isotherms, streamlines and velocity magnitudes for different aspect ratios of the enclosure so as to be able to investigate effect of aspect ratio on turbulence. It was shown that as the aspect ratio of increased from 2, 4, 6 and 8 of the enclosure, the velocity of elements decreased and the vortices became smaller and more parallel thus concluded that an increase in aspect ratio decreased the turbulence.