Abstract: The microstructures and properties of 12CrNi5MoV forged steel before and after stress relieving heat treatment are studied. The results show that the strength and plasticity of 12CrNi5MoV forged steel after stress relieving heat treatment have little change compared with those of the original sample, that the impact toughness reduces, and that obvious embrittlement occurs. The root cause of the embrittlement is that the 12CrNi5MoV forged steel contains a certain amount of P and Ni, Cr, Si, Mn and other elements that are easy to cause temper brittleness.