In this paper a new adaptive under frequency load shedding algorithm is designed. The purpose of this method is to enhance the adaptability of under frequency relays and increase the security of power system during large disturbances. A suitable method to design the proposed adaptive under frequency load shedding scheme is described in detail. After completing the design, it is tested on a real power system by modeling the network in a power system studies software. Simulation results indicate that the proposed method has an improved performance for large disturbances while its response is similar to the conventional scheme for small disturbances