Abstract: Gravel dunes are the typical form of bed load motion and one of the research focuses in river dynamics, which determines the bed resistance, the bed geometry and the bed load transport rate in alluvial rivers. Most previous studies focused on the fine sediment in the middle and lower reach of rivers or in estuaries and coast areas where the grain diameter is less than 1mm. Thus, further research should be done to determine whether gravel dunes (D>2 mm) exist in mountain rivers and how to estimate the critical condition. On the basis of field surveys at Jiuduizi and Shaojibei sites in the upper reach of the Yangtze River, it is apparent that the natural form of gravel dunes does exit in mountain rivers. A series flume experiments were carried out with nature sediment (D₅₀=1.8 mm, 5.3 mm) and coal (D₅₀=4.8 mm) and stable uniform flow conditions, to simulate the formation of gravel dunes. A formula of critical condition for gravel dunes is proposed, which is expressed in terms of the dimensionless flow power w_*, R/D and the slope S according to the energy slope, the average water depth, the discharge and other parameters of the flume experiment. The discriminate coefficient of gravel dunes G_Dcr is also proposed to determinate whether dunes exist on gravel transport belt. The study can provide a reference for the regulation of channel activity, flood defense preparations and bed load transport predictions.