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The Band-e-Golestan drainage basin with a surface area of about 320 km2 is located southwest of Mashhad. This catchment area consists of three subbasins: Mayan, Dehbar and Jaghargh. Rivers in this basin flow toward the northeast and are connecting to Kashafrud River. These rivers are single channel and consequent with the bedload sediments. In many gravel-bed rivers, the grain size changes as exponential toward downstream. Most studies on the cause of downstream fining in the rivers emphasize hydraulic sorting as the dominant fining mechanism. Grain size variations and causes for fining downward were studied at 57 sites. Compositionally, gravels in the study area consists of phyllite, slate and quartzite. Consequently, These sediments are poorly sorted, positively skewed and mainly platykurtic. In the upstream, most of the boulders are phyllites while in the downstream, slate makes up the framework of gravel. The percentage of abrasion in slate is about 25.5 while that of the phyllite is about 28 (Estimated by a Los Angles Machine). Grain size distribution shows a weakly bimodality in the upstreams and becomes strongly bimodal toward downstream. Finally, at the end reach, three modes appeared, two are gravelly and one is sandy. The main factors in downstream fining of grain size in the study area, are: a) the rapid reduction of slope and b) selective transport (Hydraulic Sorting) due to changing of hydraulic condition, controlled by hydrology of drainage basin as well as catchment slope.