Data on the resistance of rocks to erosion and weathering and their voluntary ratings are key information in erosion and sediment studies. Despite the importance, rock resistance to weathering is not well studied in Iran and the conducted studies are more qualitative rather than quantitative. In this study rock mass resistances of Asmari formation (limestone) and Fars formations including Mishian (marl limestone), Aghajari (Calcareous sandstone) and Gachsaran (anhydrite) were assessed using rock mass rating (RMR) method in Behbahan, Gachsaran, Deylam and Dehdasht regions. ANOVA and Tukey,s analysis of resistances of 81 collected rock samples showed there were significant differences (P<0.05) between geological formations in term of rock resistance. According to RMR rating, rock mass resistances were in order of Asmari limestone> Mishan limestone> Aghajari calcareous sandstone> Gachsaran anhydrite. Results also indicated there were no significant differences between rock resistances of Fars formations but rock resistance of Fars formations differed significantly from Asmari formation.