More effective nondestructive technology for the estimation on material properties has been sought. In this research, a new electrical resistivity method was attempted for the estimation of the fracture toughness of a degraded turbine rotor steel. 1Cr-1Mo-0.25V turbine rotor steel specimens with seven different periods of aging were prepared by an isothermal heat treatment at 630degreesC and the electrical resistivity was determined by a direct current four points potential method. The electrical resistivity at room temperature monotonously decreased with the extent of degradation of the material. It was also observed that the fracture toughness was correlated with the electrical resistivity. The microstructural changes of material during aging was examined by X-ray diffractometer and electron probe micro analyzer (EPMA). A larger amount Mn23C6 and (Cr2.5Fe4.3Mo0.1)C-3 appeared in grain boundaries of the more heavily aged material.