The interannual fluctuations of atmospheric CO_2(IFAC),as an important signal of global carbon cycle research,are closely related to the El Nino-Southern Oscillation(ENSO).However,most studies regarding the impact of volcanic activities tend to focus merely on the post-Pinatubo eruption.By using monthly observational data of atmospheric CO_2 and delta~(13)C at Mauna Loa and the South Pole from the period of 1959 to 2008,this study analyzes the features of IFAC.A positive correlation is found between IFAC and ENSO,and a negative correlation between IFAC and the volcanic eruption index.The massive eruptions could reduce the impact from strong ENSO on IFAC.The influence of volcanic activities is related to the eruption intensity,as well as the magnitude of eruptions and duration of eruption effects.ENSO and volcanic eruption work together on the IFAC.During the analysis period,the diversity of atmospheric CO_2 and delta~(13)C variations after El Chichon and Pinatubo eruptions are related to the relative beginning time of both events.Using delta~(13)C analysis and Keeling Plot calculation,ENSO exerts influence on IFAC through the changes of terrestrial biosphere,along with volcanic eruption,by means of oceanic uptake enhanced by the cooling and fertilization effects.