Abstract: From the moment-independent importancemeasure of basic variable effect on response distribution, an importancemeasure is defined to describe the effect of basic variable on failureprobability. Compared with the importance measure of the basic variableeffect on the response distribution, this measure can directly provide the globaleffect of the variables on the structural safety, so can providemore guidance in engineering design than the former. The basic properties ofthe importance measure of the basic variable effect on the failureprobability are discussed in this paper. Since the saddlepoint based linesampling method is independent of the distribution types of the randomvariables, and has the advantages of high accuracy and efficiency, thismethod is employed to estimate the defined importance measure.