Artificially roughened solar air heaters perform better than the plane ones under the same operating conditions. However, artificial roughness leads to even more fluid pressure thereby increasing the pumping power. Roughness and flow parameters viz. relative roughness pitch p/e, relative roughness height e/D and flow Reynolds number Re have a combined effect on the heat transfer as well as fluid pressure (friction factor). Investigation for the optimal thermohydraulic performance (i.e. maximum heat transfer for minimum friction loss) of artificially roughened solar air heaters has been carried out. An optimisation parameter known as roughness Reynolds number which combines the roughness and flow effect and is expressed as e(+) = e/D root f(r)/2 Re has been considered. Thermohydraulic performance has been defined by the equation n(thermo) = (St(r)/St(s))(3)/(f(r)/f(s)). It has been found that e(opt)(+) similar or equal to 24 gives the optimal thermohydraulic performance in such collectors and therefore the optimal thermohydraulic performance curves [3], for designing such collectors for practical applications are suitable. The Value of optimal thermohydraulic performance has been found to be about 71% corresponding to e(opt)(+) = 24.