Soil erosion is a major problem in Abay basin, Ethiopia. Gedeb watershed which found on the southern side of Abbay basin and found in south Gojjam sub-basin in Ethiopia and mostly affected by soil erosion, sediment transport and land degradation, due to Deforestation, overgrazing, over cultivated and poor land management and this problem accelerates the rate of erosion and which becomes soil fertility reduction. To overcome this problem in exposed area make it strip cropping, construct bunds, cover with plants/vegitatation at upstream of a watershed. The main objective of this study was to determine the effect of land use/cover dynamics on sediment yield on Gedeb watershed. Soil & Water Assessment Tool (SWAT) model was calibrated, validate & used to evaluate changes on sediment yield and stream flow. Two SWAT model set up were run to evaluate the impacts of land use and land cover dynamics after Downloading Landsat image, supervised classification and preparing the land use/cover map in ArcGIS software on stream flow and sediment yield for both 1985 and 2018 of the study areas. The study also to estimate sediment yield in the Gedeb watershed using SWAT model, to investigate the spatial and temporal variation of sediment yield, identify soil erosion prone areas from sub catchment. The calibration and validation were done after checking the performance of sensitive analysis. The first most sensitive parameters were SCS curve number (CN2) for both flow calibration and for sediment calibration. The model performance evolution during monthly stream flow calibration and validation period indicated that R2=0.81, NS=0.79 and R2=0.75, NS=0.747 using 1985 land use/cover map and for 2018 land use land cover map R2=0.8, NS=0.75 and R2=0.73, NS=0.72 respectively. For Sediment calibration the coefficient of determination and Nash-Sutcliffe coefficient give as R2=0.68, NS=0.62 and R2=0.65, NS=0.63 by using 1985 land use/cover and by using 2018 land use/cover R2=0.71 NS=0.69 and R2=0.69, NS=0.63 respectively. The annual average measured suspended sediment generated from the sediment rating curve was 16.48 ton/ha/yrs. and the simulated annual average sediment yield by SWAT model was 12.14 t/ha/yr for 2018 land use map and for 1985 land use map 9.7t/h/y. Hence the result showed that the watershed affected by land use land cover change or cultivated land has expanded during the study period of 1985 2018. i.e. The range and bush land, forest land was highly expanded to agriculture. The catchment is the most erodible area and the average annual sediment distribution ranges from greater than 10 ton/ha for 2018 land use land cover. Generally, sub basin 6, crop agriculture, soil pellic vertisols, is mostly affected by runoff and sediment yield at HRU 51.