In order to determine the amount of under voltage load shedding（UVLS）， Markov method is used to calculate the probabilities of various operations of UVLS schemes. Based on this approach， and considering voltage collapse risks， the voltage collapse risk index can be constructed. This risk index uses the amount of load shedding as variable under expected contingency cases. Using the minimum of this risk as criteria， the amount of load shedding can be determined. Different with traditional methods， this one is obtained based on a full-scale contingencies analysis and consideration of under voltage load shedding scheme reliability. The calculation information is more comprehensive and the result is more accurate than traditional ones. IEEE 14-bus example reveals the effectiveness of this method. It can provide a useful reference for the design of under voltage load shedding schemes.