Simulations from six climate system models are analyzed to investigate the ENSO amplitude changes in response to a transient increase of the atmospheric CO2. It is found that the ENSO amplitude amplifies as the CO2 concentration increases, then weakens once the concentration is held fixed at 2xCO2 level in the Meteorological Research Institute (MRI) model simulation, but with no significant sensitivity in the other five models. By comparing the MRI and the Geophysical Fluid Dynamical Laboratory (GFDL) models, both of which simulate ENSO reasonably well for the unperturbed climate, we investigate the origin of the sensitivity or lack of sensitivity of the ENSO amplitude to changes in atmospheric CO2 concentration. It is found that in the MRI model the oceanic sensitivity to the equatorial zonal wind stress is significantly decreased in the doubled CO2 period compared to the transient period. Moreover, the vertical stratification at upper levels is weaker in the former period than in the latter period. However, these changes in the GFDL model are negligible.