Intrinsic defects and Mo-related defects in vanadium carbide VC, as well as the defect complexes between vacancies and Mo defects were investigated by means of first-principles calculations within the framework of density functional theory. In addition, Mo diffusion in VC was also studied using LST/QST method. The formation energies of defects have clearly shown that except C vacancy (V-C) all other point defects are not energetic favorable compared to perfect VC. V(C)can exist in the lattice forming nonstoichiometric carbide VCx(x < 1), and also can stabilize the Mo-related defects (SMo-V, SMo-C, and T-Mo). Free Mo atoms have the strong tendency to enter the already formed V(V)and occupy the lattice position of V atoms. Meanwhile, Mo atom in C lattice (SMo-C) and interstitial Mo (I-Mo) atom can also enter the V(V)position stabilizing the lattice structure. SMo-C + V(V)will transform into SMo-V + V(C)and I-Mo + V(V)will transform into S(Mo-V)during optimization, and large binding energy makes Mo atom tend to exist in the interstitial position. From the perspective of energy, Mo atom tends to diffuse through the interstitial position.