The steady natural convection heat transfer in an air filled enclosure, which is composed of the rectangular and parallelogrammic portions, has been studied numerically and experimentally. The air-filled enclosure consists of vertical heat source and sink walls, and adiabatic plates at the top and bottom walls. With the constant geometrical aspect ratios, the effects of guide vane installed in a composed enclosure having the various dimensionless channel depths, d*, are studied to find the enhancement of heat transfer of inner diode to a certain extent. The governing equations for two dimensional, laminar, natural convection process in an enclosure are discretized by the control volume approach which insures the conservative characteristics to be satisfied in the calculation domain, and solved by a modified SIMPLE algorithm. Computations are carried out for Prandtl number Pr=l, and the constant modified Grashof number Gr* = 1.4×1010 and φ = 45°. In order to compare the numerical results with experimental one, experimental apparatus are composed. To get the greater understanding of the flow and heat transfer behaviors, flow patterns with streamlines and isotherms at different dimensionless channel depths are shown. Experimental results showed that for given modified Grashof number the increasing size of a guide vane in the composed enclosure enhances the heat transfer rate.