Using a terrestrial ecosystem model (Sim-CYCLE), CO2 exchange between the biosphere and the atmosphere was simulated for the period of 1980–2000. As a result, the biosphere was totally a sink of CO2, and the phase of the interannual variation of the global annual CO2 flux from the biosphere was similar to that of the annual increase of the global atmospheric CO2. The interannual variation of the global annual CO2 flux showed high correlation with that of the annual mean ENSO index (NINO.3 monitoring index) for a time lag of 1 yr. As for the regional annual carbon fluxes, some showed high correlation with the annual mean ENSO index for a time lag of 1 yr and some without time lag. Among the regional carbon fluxes those showed high correlations with ENSO index without time lag, some showed positive correlations and some showed negative correlations, and they canceled out each other in total. These negative correlations were found in tropical regions. All the regional carbon fluxes those showed high correlations with ENSO index for a time lag of 1 yr showed positive correlations.