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In situ particle size and settling velocity distributions of suspended sediment in the Pearl River Estuary were directly measured by LISST-ST during wet and dry season cruises. Preliminary observations and experimental data show complex temporal and spatial variations in estuarine flocculation. The in situ median diameter of the suspended sediments varied from 10 to 96 μ m . The measured in situ settling velocity of different sized particles ranged from 0.001 to 0.02 cm s - 1 and appeared to be described by the formula: ω I = kd i n ( 0.29 n 0.82 ) . The low suspended sediment concentration had a minor effect on the flocculation of suspended matter in the estuarine water. The salinity level gradient and vertical stratification were two important factors in the formation of estuarine flocs during the wet season cruise. The suspended particles had a different response to salinity during different water quality conditions during the dry season cruise. In seriously polluted estuarine waters, a much higher settling velocity occurred because the flocs become denser. We also found in situ particle size and settling velocity in the Pearl River Estuary was much smaller than in other estuaries, and various reasons are discussed.