Abstract: A non equilibrium statistical theory of thermal fatigue which connects microscopic mechanism with macroscopic properties is established. A physical picture and a microcrack evolvement equation are given. With the help of a formula of thermal strain and dislocation mechanism of microcrack evolvement, a microcrack growth rate and a probability density distribution function are found. The formula of mean length varying according to the number of thermal circles, which is in agreement with the experiment result on the whole, is given also. Furthermore, a formula of thermal fatigue lifetime is got through the formula of thermal fracture. The impact of the temperature difference and Young's module on the thermal fatigue lifetime is made out finally.