Evolution of M23C6 carbide during isothermal aging of the 10% Cr martensite ferritic steel at 650 °C has been investigated by mainly using SEM-BSE and TEM-EELS. It was found that the M23(C,N)6 carbonitride formed after aging for 27,000 h since nitrogen atoms replacing part of carbon atoms dissolved into the M23C6 carbide. Such M23(C,N)6 carbonitride exhibited high stability as compared with previous M23C6 carbide. The size of M23(C,N)6 hardly changed even if the duration of aging exceeded 36,700 h, keeping at the level of ~200 nm. This suggests that the nitrogen could suppress the coarsening of M23C6 carbide once it dissolved into the M23C6 carbides. Moreover, the hardness of the10% Cr steel remained stable during aging from 27,000 h to 36,700 h, attributing to the high stability of the M23(C,N)6.