Atmospheric CO2 and China's climate have changed greatly during 1961-2000. The influence of increased CO2 and changing climate on the carbon cycle of the terrestrial ecosystems in China is still unclear. In this article we used a process-based ecosystem model, Biome-BGC, to assess the effects of changing climate and elevated atmospheric CO2 on terrestrial China's carbon cycle during two time periods: (1) the present (1961-2000) and (2) a future with projected climate change under doubled CO2 (2071-2110). The effects of climate change alone were estimated by driving Biome-BGC with a fixed CO2 concentration and changing climate, while the CO2 fertilization effects were calculated as the difference between the results driven by both increasing CO2 and changing climate and those of variable climate alone. Model simulations indicate that during 1961-2000 at the national scale, changes in climate reduced carbon storage in China's ecosystems, but increasing CO2 compensated for these adverse effects of climate change, resulting in an overall increase in the carbon storage of China's ecosystems despite decreases in soil carbon. The interannual variability of the carbon cycle was associated with climate variations. Regional differences in climate change produced differing regional carbon uptake responses. Spatially, reductions in carbon in vegetation and soils and increases in litter carbon were primarily caused by climate change in most parts of east China, while carbon in vegetation, soils, and litter increased for much of west China. Under the future scenario (2071-2110), with a doubling CO2, China will experience higher precipitation and temperature as predicted by the Hadley Centre HadCM3 for the Intergovernmental Panel on Climate Change Fourth Assessment. The concomitant doubling of CO2 will continue to counteract the negative effects of climate change on carbon uptake in the future, leading to an increase in carbon storage relative to current levels. This study highlights the role of CO2 fertilization in the carbon budget of China's ecosystems, although future studies should include other important processes such as land use change, human management (e.g., fertilization and irrigation), environmental pollution, etc.