This paper reports that the ratios of double to single electron loss cross-section (R) of O2+ in collision with Ar and He at the velocity of 1-4ν0 (ν0 is the Bohr velocity) have been obtained by the coincidence technique. The trend of R-V in the experiment indicates that the effective charge varies with injected velocity. The effective charge can be obtained by the n-body classical trajectory Monte Carlo method, which is interpreted by the molecular Coulomb over barrier model.