The equilibrium geometries,harmonic vibrational frequencies,natural bond orbital(NBO)charge distributions,and the affinity energies of Li⁺ attachment to CH_3-nF_nO₂(n = 0～3)have been investigated using ab initio molecular orbital theory and density functional theory(DFT).The structures were determined using unrestricted second order M?ller-Plesset(UMP2)and unrestricted Beck's three-parameter Lee-Yang-Parr(UB3LYP)theories in conjunction with 6-31G(d,p),6-311G(d,p),6-311+G(d,p),6-311G(2d,p)and 6-311+G(2df,2p)basis sets,and the stretching vibrational frequencies were investigated for CH_3－nF_nO₂ and CH_3－nF_nO₂Li⁺,using optimized geometries at the same levels. The calculated vibrational frequencies of radicals at UB3LYP/6-311G(d,p)levels were consistent well with the experimental data. The affinity energies of Li⁺attachment to CH_3－nF_nO₂(n = 0～ 3),calculated by DFT and ab intio are all large which suggest that CH_3－nF_nO₂Li⁺ complexes could be possibly detected as stable species in gas phase by ion attachment mass spectrometry.