The chlorophyll a concentration and grain size in the sediment of an intertidal flat, as well as their relations were studied based on experiment data observed in the mudflat of Chongming Dongtan in Changjiang Estuary, where samples from three transects were taken during April, May, October and December in 2005. The results showed that there were remarkable seasonal variations of chlorophyll a concentrations and grain size at the surface of the mudflat, i.e., the chlorophyll a concentrations were higher in April ［(5.997±0.803 ) μg·g-1］ than in December ［(0.571±0.091) μg·g-1］, and the grain size had the similar trend of seasonal variations. The correlation between chlorophyll a concentrations and finegrained sediment content was significant at the 0.05 level ( r=0.693, P=0.012). Moreover, due to the light limitation, the chlorophyll a concentrations at the surface of the sediments were high, and rapidly decreased with the depth, and also were higher on the sediments surface in bare tidal flat than in Scripus mariqueter zone of the transition area.