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In this paper, effects of runback ice accretion on NACA 23012 airfoil have been studied in an experimental and numerical manner. According to this purpose, experiments have been done using runback ice within Reynolds No. 0.6 x 106 over the angle of attack from 0 to 20 degrees and results have been compared with the clean airfoil results. Having examined behavior of the flow pattern and aerodynamic coefficients of the iced airfoil, results were compared with those of the clean airfoil. It can be concluded that icing phenomenon affects aerodynamic performance of the airfoil in two ways; first, it occurs at low angles of attack prior to stalling of the airfoil and the effect is local. In this case ice accretion on the airfoil contributes to formation of the flow separation bubble behind the ice ridge on the upper surface of the airfoil. After numerical simulation of flow field, flow separation bubble behind the ice ridge was observed. The main effect of icing which is related to the second way occurs at angles of attack close to stall and post-stall. In this case flow pattern around the airfoil as well as aerodynamic coefficients undergo fundamental change. ln addition, it was clear runback ice causes stall angle to decrease 2 degrees and maximum lift is reduced about 8 percent. [ABSTRACT FROM AUTHOR]