The study investigates the threshold condition for the initiation of cohesionless sediment transport on a nonhorizontal streamwise slope. Theoretical study of the stability of a sediment particle lying on a non-horizontal bed slope shows that the critical shear stress is a function of the streamwise bed slope. Experiments conducted with a closed-conduit flow show that the equation derived from force analysis adequately evaluates the critical shear stress of sediments lying on slopes ranging from steep positive to negative (adverse). Tests conducted with a closed-conduit flow avoid the problems associated with those conducted with an open channel flow. In the latter flow condition, uniform flow is often difficult to achieve in a steep channel, and impossible to achieve when the streamwise bed slope is adverse.