Natural convection in closed cavities is one of the most important subjects of fluid flow and heat transfer. This subject has received much attention in last ten years due to many energy-related applications, such as cooling of electronic components, thermal insulation of buildings using air gaps, solar energy collectors, semiconductor production, furnaces and fire control in buildings [1]. The motion of the fluid in natural convection occurs due to the buoyancy forces imposed on the fluid when its density of the heat transfer surface is described as a result of thermal expansion of the fluid in a non-uniform temperature distribution. Most authors considered the flow and heat transfer inside a square or rectangular cavities because the numerical solution is not complicated and the geometry is very simple to simulate. But, natural convection in non-rectangular cavities such as triangular and trapezoidal cavities has received little attention even in the recent time. The flow inside these cavities is much more complicated to investigate, as the boundary zone and the middle core zone never have the same effect for a certain boundary condition considered. On the other side, actual or real cavities occurring in practice generally have the shapes different from square or rectangular shape.