A stochastic model for predicting fatigue crack growth was discussed. The model is based on the results of fatigue test using relatively large CT specimen (300mm wide, 18mm thick) under constant stress intensity factor range, Δ K , condition. It is written as da / dN = C _o C ( x ) ( Δ K / K _o) ^{m ( x )}, where m ( x ), C ( x ) represent large scale variation and small scale variation, respectively. Numerical simulation for fatigue propagation life under constant load amplitude conditions were conducted. The major results are: (1) There are large scale variation and small scale variation of material properties of fatigue crack growth. (2) It is very important to model the statistical correlation of material properties along crack path, particularly when the correlation length is large. (3) The mean and variance of N ( a ) are represented as a function of mean, variance and autocorrelation of m ( x ).