Under frequency load shedding (UFLS) technology is widely used in power system. It significantly ensures the stability of the power system by means of shedding excessive loads in certain places. In order to improve the traditional UFLS scheme, many researchers are taking the frequency rate of change as indicator of determining the shortage of the active power in the power system. In this research, a new function is created to calculate the power imbalance by analyzing the factors that influence the measure of the frequency change rate as well as the related problems in the application of the frequency change rate. Moreover, a new method that utilizes the voltage sensitivity to determine the location and the corresponding quantity of the load shedding is proposed. The simulation results show that the new approach is superior.