The different oxidation behavior of TiC and VC(100) surfaces by molecular oxygen has been investigated by density functional theory with a slab model. From the thermodynamic stability of the final states that involve dissociated O(2), one cannot well explain the experimental observations. Two different oxidation pathways of TiC and VC(100) surfaces have been explored in this work, and the results indicate that two channels share the same precursor state. However, from the precursor, only the pathway leading to the formation of a C-O bond is energetically feasible for the TiC(100) surface, while on VC(100) the O atoms tend to occupy the metal surface sites due to a smaller energy barrier for this channel. Further band structure calculations reveal that the additional d electron of V atom favors the stability of the molecularly adsorbed species. The oxidation mechanism unveiled from the present calculations clearly evidences that the kinetic effects introduced by one additional d electron of the V atom play a crucial role in explaining the different surface chemistry between TiC and VC (100) surfaces.