Abstract: In order to study thermal fatigue, the microscopic mechanism is combined with the macroscopic characteristic based on the non equilibrium statistical theory of fatigue fracture. From the mixed model of Raj and Baik's, the crack growth rate, the transportation growth rate and the fluctuation growth coefficient are derived. Subsequently, the probability density function of microcrack, the average length of microcrack are found from the stochastic microcrack propagation equation. By a further analysis, the probability of fatigue fracture and the reliability of metal in the field of cyclic stress are achieved. At last, some discussion about results are made.