The growth of the fatigue crack under fluctuating load of which amplitude was assumed to be a random variable following Rayleigh distribution was discussed. The probability distribution function of the fatigue crack length after a number of load applications was given and the momentary risk and reliability functions were obtained using Griffith-Irwin criterion for catastrophic failure. Numerical simulations were carried out to check the result and compare it with Palmgren-Miner rule. It was shown that the fatigue lives estimated from Palmgren-Miner rule were much longer than those of the present model. The result was applied to a multiple-member structure to see its fail-safe capacity. Two methods were proposed to determine upper and lower bounds of reliability function of the structure.