Abstract: The basal friction characteristics of granular materials are quite different from those of common solid. We carried out a series of sliding friction tests to investigate the effect of grain size distribution(i.e. particle size and mass percentage) on the basal dynamic friction coefficient of granular materials. Based on the analysis of video images and curve fitting method, the fitting function of displacement and time was established, which was used to deduce the dynamic friction coefficient by the analysis of acceleration in the sliding process. Experimental results indicate the following characteristics. The basal dynamic friction coefficient of granular materials decreases linearly with time. For monodisperse granular materials, the basal dynamic friction coefficient of fine particles is higher than that of coarse particles. For bidisperse granular materials, the basal dynamic friction coefficient of granular materials decreases sharply until reaching a minimum value(content of fine particles, CFP≤40%), then increases sharply(CFP 40% ~60%), and increases slowly with further increasing content of fine particles(CFP≥60%).