The electronic structures of MC compounds (M=3d-transition-metal) in NaCl structure have been studied by using the density functional theory and the calculated results indicate that the arrangements of the band structures and total density of states of MC compounds are similar. However, the chemical bondings of these compounds are different. According to the components of the energy bands crossed by EF, the MC compounds can be approximately divided into three groups, ScC, TiC and the other MC compounds, respectively. By using two-sublattice model, the bonding energies of M–C, M–M and C–C bonds for MC compounds in NaCl structure are determined quantitatively. The results show that M–C bond has the largest contribution for the cohesive energy of each MC compound and the TiC and VC have the strongest M–C bond, and M–M bond, respectively. For those metastable MC compounds, the interaction between carbon atoms cannot be neglected. Furthermore, our calculated cohesive energies of MC compounds in NaCl structure are in agreement with the experiments.