Theoretical and experimental investigations are presented on the threshold of noncohesive sediment motion under a steady-uniform stream flow on a combined transverse (across the flow direction) and longitudinal (streamwise direction) sloping bed. Theoretical analysis of the equilibrium of a sediment particle lying on a combined transverse and longitudinal sloping bed shows that the critical shear stress ratio (ratio of critical shear stress for a sloping bed to that for a horizontal bed) is a function of the transverse bed slope, longitudinal bed slope, angle-of-repose of sediment particles and lift-drag ratio. As in laboratory flume study, the uniform flow is difficult to establish in a steeply sloping channel and is impossible to obtain in an adversely sloping channel, experiments were conducted in two inverted semicircular ducts (closed-conduit flow) for three sizes of uniform sediments. The critical shear stresses for experimental runs were determined considering side-wall correction. The experimental results correspond closely with the results obtained using the present model. The model can be applicable to both the channel beds and banks.