Abstract: The time-series surface sediments were collected monthly in high flat phragmites australis zone, middle flat scirpus triqueter zone and low flat zone, respectively, at Chongming tidal flat which lies in Changjiang Estuary during July 2010 and February 2011.By using a water elutriation apparatus, sediments were separated into four size fractions from 8~16 μm to >63 μm which were called A (the coarsest), B, C and D (the finest).In order to know the distribution of ⁷Be in different size fractions and different seasons, sediment grain size was measured as well as ⁷Be activity.The following conclusions were drawn:In the same level the coarse-particle fraction exhibited an offshore increasing trend, and the fine fraction showed a decreasing trend.The activities of ⁷Be generally decreased as the grain size increased.Correlation analysis between grain size and ⁷Be activity showed that the ⁷Be activity of sediment in high and middle tidal flat were influenced by the precipitation more than that of the low tidal flat.The correlation between grain size and ⁷Be activity at station 14# was weakest among all the sampling stations.However, there was no obvious seasonal difference for the correlation between grain size and ⁷Be activity.Our study showed that the grain size effect of ⁷Be activity should be considered when ⁷Be is utilized to trace transport and deposition of particles in the estuary and coastal areas on a seasonal scale.