Abstract: Flocculation is the primary cause of fine sediment deposition in the Three Gorges Reservoir (TGR),and the critical conditions of flocculation is of great significance to the law and simulation of the sedimentation in the TGR. Field measurements of sediment flocculation were carried out at the Zhongxian and Fengjie reaches in the TGR during the flood season. Based on the acoustic backscatter intensity from ADV and sediment sampling,the instantaneous flow velocities and sediment concentrations were measured synchronously. The sediment settling velocities and floc sizes were calculated using the sediment diffusion theory. The relationships between flocculation degree and particle size,flow velocity and sediment concentration were obtained,respectively. Results show that the sediment flocs are mostly in the region of middle and light flocculation,but less in the region of heavy flocculation. The critical particle size of the sediment flocs is about 0.018 mm,the critical flow velocity is about 0.7 m/s,and the critical sediment concentration is about 0.8 kg/m³. This research can provide theoretical supports for the basic law of sediment transport and simulation of sedimentation in the TGR.