In this study, numerical analysis was used to investigate the effect of different parameters on thermal efficiency of flat plate solar collectors. Various geometries were examined in order to assess the influence of geometrical characteristics and operating conditions on thermal efficiency of solar collectors. Important parameters such as absorber thickness, riser position, shape of tube cross section, absorber material, absorber absorptivity, glass transmissivity, and mass flow rate have been investigated. Results show that the efficiency of collector with risers on top of the absorber plate is 4.2% more than that of the collector with risers on bottom. Also the tube cross-sectional geometry shows strong effect on the efficiency e.g. the efficiency of collectors with circular tubes is 38.4% more than that of collectors with triangular cross sections. Thermal efficiency of solar collectors increases with increasing the fluid flow rate, plate absorptivity, absorber thickness, and glass transmissivity.