There are different methods for controling the flood damages which one of them is flood zoning. In this method, flood zones are determined by simulating river hydraulic behavior and its flood plain for different return periods.In other hand, soil and specially plant coverage are the most effective factor to determine Manning roughness coefficient. So it is expected, seasonal plant coverage changing affect on Manning roughness coefficient and flood hydraulic behavior forecasting. In this research a section of Haraz River (3250 m in upstream of Amol, northern Iran) was studied to find the affect of the season in determination of Manning roughness coefficient in predicting flood hydraulic behavior because of vegetation changes. Geographical Information System (HEC-GeoRAS Extension ), HEC-RAS software and river topographic maps in 1:500 scale were used in order to simulate Haraz River’s hydraulic behavior and its flood plain.The Manning roughness coefficient was determined in winter and summer using Cowen method for corss sections separately.The quantitative and qualitative results showed that season of the determination Manning coefficient causes changes on n values on left and right banks of the river and flood plain because of vegetation changes. Consecuently, that causes some changes in flood zones, water velocity and depth, shear stress and the distribution of the flood volume in river channel and its flood plain for different return periods.