This paper presents an experimental investigation of the noncohesive sediment threshold under a unidirectional steady-uniform stream flow on streamwise sloping (down slope and adverse) sedimentary beds. The theoretical analysis of the equilibrium of a solitary sediment particle lying on a sloping sedimentary bed shows that the threshold shear stress ratio, that is, the ratio of threshold shear stress for sloping bed to that for plane bed, is a function of the streamwise bed slope and angle of repose of sediment particles. Experiments carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments show that the equation derived from theoretical analysis corresponds closely to the experimental data. In an open channel flow (laboratory flume study), uniform flow is a difficult, if not impossible, proposition for a steeply sloping channel and is impossible to obtain in an adversely sloping channel. To avoid this problem, the tests were conducted with a closed-conduit flow. This paper presents an experimental investigation of the noncohesive sediment threshold under a unidirectional steady-uniform stream flow on streamwise sloping (down slope and adverse) sedimentary beds. The theoretical analysis of the equilibrium of a solitary sediment particle lying on a sloping sedimentary bed shows that the threshold shear stress ratio, that is, the ratio of threshold shear stress for sloping bed to that for plane bed, is a function of the streamwise bed slope and angle of repose of sediment particles. Experiments carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments show that the equation derived from theoretical analysis corresponds closely to the experimental data. In an open channel flow (laboratory flume study), uniform flow is a difficult, if not impossible, proposition for a steeply sloping channel and is impossible to obtain in an adversely sloping channel. To avoid this problem, the tests were conducted with a closed-conduit flow.