Models of oxidized Si (111) surface under different C coverage were established to study the charge transfer ability of Si nanomaterial from strategy of C surface modification using first-principles calculation. The calculated formation energies show that structures of C surface-modified oxidized Si (111) surface are stable. The electronic properties present that the interaction between C and Si atoms is mainly contributed by the hybridization of C-2p and Si-3p states. And the interaction between C and Si atoms increases firstly and then decreases with the increasing C coverage rate. The transfer charge between C and Si reaches a maximum when C coverage rate is 0.5. We speculate that the conductivity of experimentally prepared Si nanomaterial does not monotonously enhance with the increasing C concentration, which would reach a maximum at a certain C concentration, and then decreases.