Four tidal flat sediment cores were collected from east to west along the northern shore of Chongming Island and specific activities of ¹³⁷Cs, ²²⁶Ra and ²¹⁰Pb were determined. The sedimentation rates of the Chongming Island mudflats were calculated by means of ²¹⁰Pb and ¹³⁷Cs geochronology. For the east and west ends of the northern mudflats, the sedimentation rates by ²¹⁰Pb geochronology were 3.08 cm/a and 2.34 cm/a, respectively, while the sedimentation rates by ¹³⁷Cs geochronology were 6.19 cm/a and 2.06 cm/a , respectively. Although the sedimentation rates calculated by ¹³⁷Cs are generally higher than those from ²¹⁰Pb, the same feature of sedimentation rates, viz “faster in the east, slower in the west” was indicated. And vertically the sedimentation rates were gradually slowdown from the bottom to the top since 1954.