Taking the hot working die steel (HWDS) 4Cr3Mo2NbVNi as an example, the phase electron structures (PES) and the biphase interface electron structures (BIES) of Mo2C and V4C3, which are two kinds of important carbides precipitated during tempering in steel were calculated, on the basis of the empirical electron theory of solids and molecules and the improved TFD theory. The influence of Mo2C and V4C3 on the mechanical properties of HWDS has been analyzed at electron structure level, and the fundamental reason that the characteristic of the PES and the BIES of carbides decides the behavior of them has been revealed.