The thermoelastoplastic stresses caused within a low-alloy steel rotor shaft material in water cooling are dynamically simulated by applying the finite element method and the incremental strain theory. The thermal stress concentration at the fillet of the model with thermal constants and mechanical properties varied due to temperature is explained according to contour lines of stress. Then, it is considered that there are sufficient capabilities for the fracture in applied positions, though the equivalent stresses exceed the yield point under the proper temperature at both surfaces of the fillet and longitudinal center of shaft. As the result, the variable principal stress ratios at the fillet are obtained through comparison with the tangential stress at the surface in the middle of the shaft, which rapidly increases in the beginning of cooling.