The selective Co capping is essential for improving the electromigration resistance of the copper interconnects. We studied the mechanism of selective Co deposition using CpCo(CO)2 on Cu over SiO2 by density functional theory calculation. We also investigated the role of H2 co-reactant and the impact of the silylation treatment of SiO2. The calculation results show that H2 plays a critical role on the Cu substrate but acts as a spectator on the SiO2 substrates.