Sand and gravel mining from the bed and banks of river causes change in the hydraulic, hydrological and current carrying capability conditions and increases erosion of river bed and banks and degradation of existing structures is in the river. Awareness of the impact of sand and gravel mining on the amount, quality and sediment transport is inevitable. The present study was therefore formulated to evaluate the effect of sand and gravel mining from the Kojour River at Educational and Research Forest Watershed of Tarbiat Modares University in Mazandaran Province on changing suspended and bed loads quantity and the diameter changes of the bed load transport. Towards this attempt, 12 and 3 samples were taken before and after sand mining at the main outlet of the watershed, respectively. The suspended load samples were taken using bottle samplers and bed load samples were taken with a basket-type bed load sampler. Bed load granulometery was performed using standard sieves. The analysis also was carried out in GRADISTAT and Excel 2007 softwares. The results showed that after sand and gravel mining the rates of suspended load increased from 300 to 547 %, bed load was an average of 1130 %. On average, the ratio of bed to suspended load was 120% higher than that recorded for period without sand and gravel mining conditions. The D₁₀ diameter of bed load had increased for low, moderate and intensive sand and gravel mining conditions at rate of 17, 24 and 35%, respectively. The D₅₀ also increased 21% in low and moderate removal and base flow conditions and also had complex behavior in intensive removal and different flow discharges. The D₉₀ had subtle changes in low removal and increased in moderate and intensive removal from 100 to 900%.