The present research was performed to evaluate the long-term isothermal temper embrittlement resistance and carbide behaviors. The material used in this study is SA336F22, the forged heavy wall 2.25Cr-1Mo steel. Temper embrittlement susceptibility after isothermal aging for 36,000h at 430-515℃ was assessed with the shift of the 50% fracture appearance transition temperature (FATT_(50)), 40ft-lb transition temperature (vTr_(40)) and grain boundary segregation of phosphorus by AES. XRD and TEM-EDS examined the behaviors of carbide. The shift of FATT and vTr_(40) of 2.25Cr-1Mo steel were significantly increased up to 10.000h aging then slowly saturated at 515℃. At lower temperature, however. saturation occurred slowly. The type and composition of carbides were changed to the Mo-rich stable ones(Mo-rich Mo₂C and M_6C), which reduces the Mo content in matrix. And P-segregation was detected on the prior austenite grain boundary after isothermal aging. These results suggested that the temper embrittlement be related with P-segregation caused by Mo-consuming at the matrix.