Abstract: This paper briefly introduces some researches about the reliability analysis of aircraft structures plroceeding in Northwestern Polytechnical University （NPU）. A set of fundamental equations with some numerical examples to compute the failure rate of aircraft structures in service is presented. It takes the factors into consideration as complete as possible.From the numerical results the influences of various factors on reliability of aircraft structures are estimated and the current criteria of aircraft fatigue design are evaluated. It is also pointed out that the 10-7 level of failure probability （per hour） with 95% confidence recommended in Engineering Sciences Data of RAE is unreasonable.For the data treatment of some important f actors, i. e. the probability of detection, the distribution of initial length of flaw and the distribution of klc, some new methods are developed with some new results obtained. Among them are an estimation method of the parent distribution parameters of initial life from the laboratory test as well as the detection at airports with the results of incomplete life and "ultracom-plete" life, and an modified method using the F-distribution to be able to estimate simply the flaw detection probability curve. Some large samples have been tested by the authors in order to obtain more confident distribution patterns of initial crack length and of fractuer toughness, being used to evaluate the existing results in literature.