This article is present for a detailed investigation of the design of solar air heater having rib roughness on the absorber plate by using the application of computational fluid dynamics (CFD). In this Solar air heater an absorber plate is made of ‘Al’ and roughened with transverse ribs due witch creates the turbulence in the flow of fluid (air) and increase the heat transfer from the absorber plate to the fluid. Quantity of heat transfer is depending on Nusselt number. Nusselt number depends on surface heat transfer coefficient ‘h’. Heat transfer coefficient is strongly depends on the relative roughness height (e). Here ANSYS FLUENT v 14.5 is used to simulate fluid flow through a conventional solar air heater.