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.