Solar air heaters have been used for many purposes requiring low to medium grade thermal energy, mainly due to their low manufacturing cost, simple design and operation, and maintenance. Their usefulness and the quantitative energy collection has been limited because of low thermal efficiency primarily as a result of low convective heat transfer coefficient, between the absorber plate and air leading to higher plate temperature, and thus resulting in greater thermal losses. One of several methods that have been proposed and investigated for the enhancement of heat transfer coefficient is the use of artificial roughness. Since the use of inclined and transverse wire rib roughness has shown to enhance the heat transfer rate considerably, it is worth exploring. Experimental investigation has been carried out to study heat transfer enhancement by using small diameter wire ribs on absorber plate of solar air heater duct, with the roughened wall exposed to the glazing side while the remaining three walls insulated. The roughened wall has relative roughness height (e/Dh) 0.023, relative roughness pitch (p/e) 11.1, rib height 0.9 mm, angle of attack of 90° for transverse and 45° for inclined configuration, and duct aspect ratio of 12. The air flow rate corresponds to Reynolds number between 2500 – 18,000. Results of the experimental investigation revealed that provision of artificial wire rib roughness resulted in a considerable enhancement in heat transfer and friction factor. It has been found that the heat transfer coefficient could be improved by a factor up to 2.2 and the friction factor had been found to increase by a factor of 1.2 times that of the smooth duct. The investigation emphasized that the secondary flow rolling along the inclined ribs is responsible for higher heat transfer rates. Based on minimum unit energy cost criteria a set of roughness parameters can be selected by the designer for a given operating condition. Key Words: Solar air heater, wire rib roughness, air duct, inclined ribs, unit energy cost.