Temperature, salinity, chlorophyll a (Chl-a), nitrate, and sea-air differences of CO2 partial pressure (ΔpCO2) were extensively investigated in the northern East China Sea (ECS) during seven research cruises from 2003 to 2009. The ΔpCO2 showed large intraseasonal variation in the spring and summer. In spring, the areal mean ΔpCO2 in May 2004 was almost half of that in April 2008, probably associated with differences in sea surface temperature (SST). In summer, the areal mean ΔpCO2 in August 2003 was also twice as large as that in July 2006. In addition, ΔpCO2 exhibited large seasonal variation with positive values in autumn and negative values in other seasons. The positive ΔpCO2 in autumn was ascribed to vertical mixing with CO2-enriched subsurface waters and relatively high SST in this season. The annually integrated sea-air CO2 flux in the northern ECS was -2.2 ± 2.1 mol m-2 yr-1, indicating CO2 absorption from atmosphere to the sea, which was more than two times lower than the previous estimate (Shim et al. 2007) reported for the same region. This large difference was presumably responsible for the underestimation of winter CO2 influx by Shim et al. (2007) and the large interannual variation of CO2 flux. The CO2 influx in the ECS was twice that estimated for continental shelves worldwide, suggesting that the ECS acts as a strong sink of atmospheric CO2 compared to other continental shelves.