Abstract: Carbide transformation in 12Cr1MoV steel serviced for different times was studied by using X-ray diffraction quantitative analysis after electrolytic extraction in isothermal (630 ℃) aging simulation test.And the transformation rule was concluded.The results show that carbide types in 12Cr1MoV steel unchanged during service processing at elevated temperature,including M3C,M7C3,M23C6 and MC.Total carbide content increased quickly as the steel serviced for 0~80 000 h ,and then grew stable after 80 000 h.M3C content decreased while M23C6 and M7C3 contents increased with the serviced time increasing before 80 000 h ,M3C,M7C3 and MC grew stable while only M23C6 increased a little after 80 000 h.