To find out the major factors， which affect the reliability of composite generation and transmission system， by simulating computation is of great significance for the power system design and operation. According to the level of load shedding， for the purpose of Monte Carlo reliability computation， the input set， which is constituted of elements status， is divided into two subsets. Two indices that represent the differences between both subsets are proposed. One can be calculated directly， the other is the result of simple hypothesis test. Based on these two indices， not only the contributing level of elements event to adequacy index can be evaluated， but also the importance of elements can be ranked. As a result， the bottleneck of power systems can be found. The test case shows that the proposed method and indices are effective.