Abstract: The diurnal methane flux from the supratidal flat of Qi'ao Island, Pearl River estuary was measured at a 2h interval throughout the day (24h) based on static chamber during August, 2009. Two peak-values of CH₄ flux occurred in midnight (23:00) and the next morning (9:00), with the value as high as 40.07 mg·m^-2·h^-1 and 50.99 mg·m^-2·h^-1respectively, whereas two minimum occurred in the evening (17:00) and the dawn of the next morning (5:00), with the value as low as 1.13 mg·m^-2·h^-1 and 1.59 mg·m^-2·h^-1. The concentration of SO₄^2- of pore water and tidal level were simultaneous monitored, showing obviously negative correlation with the methane flux. Therefore, the tidal dynamics is the factor leading to the variations in chemical parameters of the sediment and tidal level is the key controlling factor in diurnal variations in methane flux at the supratidal flat. The diurnal mean net methane flux from sampling sites was 962.7 mg·m^-2·d^-1, suggesting that the tidal flat of the Qi'ao Island is an important methane contributor to the atmosphere in summer. Methane concentrations of pore water range from 0.52 to 5.18 mmol dm^-3 with the maximum value occurring in the sub-surface (depth 9 cm). Total organic carbon (TOC), temperature, redox potential of sediments and SO₄^2- of pore water were measured simultaneously. The results show that the high concentration of methane match well with the high TOC in the sediment and low SO₄^2- in the pore water.