The rib enhancement of heat transfer in a duct is studied numerically and experimentally, where hot air passes through a duct (0.04 x 0.16 x 1.15 m3) with different rib arrangement. The arrangement are lower 12-rib arrangement; upper 12rib arrangement and 24 rib staggered arrangement. The staggered arrangement gives better performance than the others. Also, the angle of attack was studied for lower arrangement, three different values were tested (45°, 60° and 90°). Angle of60° gives better performance. Numerically, the three-dimension continuity, Navier-Stokes and energy equations are solved by finite volume method of flow of air through (0.04 x 0.16 x 0.6 m3). Validation of the code was performed by comparing the numerical result with the results obtained experimentally for staggered arrangement only. The agreement seems acceptable. The numerical studies were extended to study the case of cold air passing through hot ribbed duct.