On the basis of modeling with structural stochastic parameters,an idea of probability diagnosis of structral failure is presented.It regards the variation of parameter of fault structure as a perturbation on the perfect structure.Crack fault of the double decker space frame is probability diagnosed as an example.The concept of probability is firstly introduced into the field of failure diagnosis, thus we successfully make an attempt of diagnosing the structures failure from qualilative stage to quantitative stage. [WT5HZ]