In this present experimental investigation the effect of geometrical parameters of multiple arc shaped roughness element on heat transfer and friction characteristics of rectangular duct solar air heater having roughness on the underside of the absorber plate have been studied. The parameters were selected on the basis of practical considerations and operating conditions of solar air heaters. The experiments carried out encompasses Reynolds number (Re) in the range of 2200-22,000, relative roughness height(e/D) range of 0.018-0.045, relative roughness width (W/w) ranges from 1 to 7, relative roughness pitch (p/e) range of4-16 andarc angle (alpha) ranges from 30 to 75 degrees. The thermo-hydraulic performance parameter was found to be best for relative roughness width (W/w) of 5. (C) 2014 Elsevier Ltd. All rights reserved.