Based on the experimental data obtained from an undistorted,moveable-bed model for studying bedload,three methods for determining bedload grain size distribution including the statistic-theory method,maximum-diameter method and average-diameter method are compared from the view points of model and prototype respectively.The results show that from the view point of model the accuracy of statistic-theory method is low at small or large flow discharge,and the other two methods give satisfactory results at various discharges.From the view point of prototype the grain size calculated with statistic-theory method is larger than the experimental data at small discharge and is smaller at large discharge.The calculated result obtained from the maximum-diameter method is smaller than experimental result at large discharge.Whereas,the calculation results always agree well with experimental data when average diameter method is used.