Methane (CH 4) is an important greenhouse gas and oceans are net sources of atmospheric CH 4. The effects of environmental factors on the CH 4 variation during different phases of the spring algal blooms were examined during two cruises conducted in the Yellow Sea (YS) from February to April of 2009. During the pre-bloom period from February to March, low CH 4 saturation (< 134%) was observed in the surface water, except at two nearshore stations where the CH 4 levels were above 140% in March due to mixing with the coastal water. During the bloom period, CH 4 increased obviously at two bloom-tracking stations, especially at the surface with mean saturations of 140% and 170%. The increase in CH 4 concentration/saturation is thought to be the result of in situ CH 4 production. The particulate organic carbon (POC) and chlorophyll a contents were believed to be important factors that influenced the CH 4 production. In addition, the presence of different dominant phytoplankton species and the grazing pressure may have stimulated the CH 4 production by supplying potential methanogenic substrates (such as dimethylsulphoniopropionate (DMSP)). Both the incubation data and the in situ estimations further evidenced the significant influence of the spring blooms on the CH 4 production. The calculated sea-to-air CH 4 fluxes during the bloom period were not significantly higher than those during the pre-bloom period despite the bloom-increased CH 4 saturation. This is due to the variation in physical forcing (such as wind speed), which is the main driver for determining the CH 4 flux. Finally, we estimated the annual CH 4 flux in the YS as 9.0 μmol m −2 d −1; the findings suggest that the YS is a natural source of atmospheric CH 4.