Experiments were performed in a shear cell device with adjustable lower wall velocity. Glass spheres with a mean diameter of 3 mm were used as granular materials. Image processing technology and a particle tracking method were employed to measure the average and fluctuation velocities in the streamwise and the transverse directions. Because of gravitation force, the flows consist of both a solid-like region with higher and more uniform velocities in the lower test section and a fluid-like region in the upper part. The velocity fluctuations were anisotropic and were greater in the streamwise direction. By tracking the movements of particles continually, the variation in the mean-square diffusive displacements with time was plotted and the self-diffusion coefficient was determined. The self-diffusion coefficients in the streamwise direction were much higher than those in the transverse direction. The dependence of the diffusion coefficients on the velocity fluctuations and the shear rate were discussed. (C) 1999 The Society of Rheology. [S0148-6055(99)00504-0].