A numerical method, employing nonstaggered grid arrangement, is incorporated to deal with mixed convection flow in a duct with a double-step expansion at inlet. The walls downstream of the expansion are maintained at a uniform temperature. In the first series of tests, the duct is placed vertically (gamma = 0 degrees), and the buoyancy effect is examined via gradually increasing Grashof number until no converged solution is obtained. With the aid of buoyancy force the recirculating flows behind the sudden expansion attach to the two step walls when the Grashof number is large enough. In the meanwhile, reversed flow is detected in the centerline region. At Re = 56 the flow remains symmetric throughout the Grashof numbers tested whereas for Re = 114 the flow held may be symmetric or asymmetric, depending on the Grashof number. Also examined is the effect of inclination angle gamma for Ga = 1000 and 3000. In the horizontal position (gamma = 90 degrees), two recirculations exist behind the two step walls. When gamma > 90 degrees, a number of recirculations are formed on the two side walls and the main stream exhibits a wavy structure. The wavy how helps enhance heat transfer. The above results for different duct orientations are equally applicable to both heating and cooling hows.