Thin-walled cylindrical low alloy steel specimens were thermally fatigued in a pressurized autoclave. Since high and low temperature water was alternately supplied into the autoclave, the specimens were subjected to homogeneous thermal stress through the wall thickness. The thermal fatigue life was defined as the number of cycles to crack penetration to the inside of the cylindrical specimen. The thermal fatigue strength was compared with the mechanical fatigue strength performed in air and in high temperature water. If taking account of the Higuchi-Iida formula, which considers the effects of strain rate, dissolved oxygen concentration and water temperature on the fatigue life, the thermal fatigue lives of low alloy steel were found to coincide with the mechanical fatigue lives.