Abstract: The strength parameters in Griffith Theory are taken as random variables and a corresponding analytical solution about the probability of crack propagation is given. A model is developed when both load and strength parameters are taken as random variables. The influences brought about by the variation of the parameters on the probability of crack propagation are discussed by using of the two methods mentioned above. The analytical results show that the influences on the probability of crack propagation caused by the variation of the parameters are different. The influences caused by the variation of the first principal stress and tensile strength are obvious, on which more attention should be paid.