Abstract: The image processing technique was applied to measure velocity fluctuations of suspended parti cles in open channel flows. Two kinds of spherical particles were studied:plastic particles(d=0.1~1.5 mm)and glass particles(d=0.1~0.3 mm)Experimental results reveal that:(1)coarse particles pos sess greater velocity fluctuations than fine particles in the near bed region,while the scenario is completely opposite in the upper flow;(2)the velocity fluctuation of fine particles is a function of y/h,and for coarse particles,the velocity fluctuat ions relate not only to y/h but also to the shear stress;and(3)the plastic particles have greater velocity fluctuations than glass particles of the same size.