Fault Tree Analysis (FTA) is an important method of probabilistic safety assessment. The qualitative analysis of FTA can show the failure modes and weakness of system by using minimal cut set. And the quantitative analysis of FTA can calculate the probability of top event and the critical important indexes to rank of bottom events. In reliability analysis, the failure probability (i.e. the probability of top event) can be estimated by using coherent structural function, and then global sensitivity (also named important measure) analysis can be employed to determine the important input variables. Comparing the failure probability and important rank of basic events in probabilistic safety assessment with those of reliability estimation by using several FTAs of aircraft systems, we can find that the important rank of basic events will be different for the randomness of the basic events’ probabilities.