Unsteady mixed convection heat transfer of a laminar fluid flow in ventilated enclosure was studied numerically. Thebottom horizontalsurface was considered isothermal, whereas the other walls were considered adiabatic. The fluid enters into enclosure through a hole located at the bottom of the left vertical wall and exits from the top of the opposite wall. The mathematical model is governed by two dimensional conservations of mass, momentum and energy equations. The numerical simulations of the fluid flow and heat transfer were investigated with a range of Reynolds number (50-500) and Richardson number (0-10), with Prandtl number fixed at 0.71. The effect of the aspect ratio on the fluid flow and heat transfer characteristics in the enclosure was studied in three different values (0.5, 1, 1.5 ). The numerical results showed that the maximum value of the average Nusselt number was achieved at aspect ratio of 0.5. Also, the average Nusselt number increases with the increase of Richardson and Reynolds numbers for whole different cases.