A nondestructive test technique using an electrochemical method was investigated for evaluating thermal aging embrittlement of 21/4Cr-lMo steel for a nuclear pressure vessel. Agings were conducted at temperatures ranging from 400 to 500°C for up to 50 000 h. In the anodic polarization curve measured in calcium nitrate solution, a secondary peak, which is higher for embrittled materials than for the deembrittled ones, was observed in the passive potential region. Changes in the secondary peak can be correlated with the degree of the embrittlement evaluated by the shift in Charpy ductile-to-brittle transition temperature (ΔDBTT).