An ab initio computational study of the regulating function of the methyl group in the strength of the CH center dot center dot center dot O hydrogen bond (HB) with XC equivalent to C-H (X = H, CH3, F) as a HB donor and HOY (Y = H, CH3, Cl) as a HB acceptor has been carried out at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. The bond lengths, interaction energies, and stretching frequencies are compared in the gas phase. The results indicate that the methyl substitution of the proton acceptor strengthens the CH center dot center dot center dot O HB, whereas that of the proton donor, weakens the C HB. NBO analysis demonstrates that the methyl group of the proton acceptor is electron-withdrawing and that of the proton donor is electron-donating in the formation of the CH center dot center dot center dot O HB. The electron-donation of the methyl group in the proton acceptor plays a positive contribution to the formation of the CH center dot center dot center dot O HB, whereas the electron-withdrawing action of the methyl group in the proton donor plays a negative contribution to the formation of the CH center dot center dot center dot O HB. The positive contribution of methyl group in the proton acceptor is larger than the negative contribution of methyl group in the proton donor.