In this paper, Effects of runback ice accretion on NACA 23012 airfoil have been studied experimentally and numerically. For this purpose, experiments were applied on runback ice within Reynolds number of 0.6×〖10〗^6 over the angle of attack from 0 degree to 20 degree and then results were compared with the results of clean airfoil. Generally, Having examined behavior of the flow pattern and aerodynamic coefficients of the iced airfoil the results of which were compared to that of the clean airfoil, it can be concluded that icing phenomenon affects aerodynamic performance of the airfoil in two ways; in the first way that occurs at low angles of attack prior to stalling of the airfoil the effect is local .In this case ice accretion on the airfoil contributes to formation of a 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 a fundamental change. In addition, it was made clear that runback ice causes stall angle decreases 2 degree and maximum lift reduces about 8 percent.