The aim of this study is to investigate the effect of shape and size of solid particles on their initiation of motion in open channel flows. Initial motions of 22 solitary particles having different shapes and sizes were observed in a tilting flume of rectangular cross section. A smooth fixed bed and an obstructing element of smaller height with respect to the particle size was used throughout the experiments. The ratio of the height of the obstructing element to the height of the particle was kept constant at 1/5. By either changing the slope of the tilting flume or the discharge, or both, a range of shear stress values was obtained. Various equations and graphical representations in terms of dimensionless bed shear stress, grain Reynolds number, and the ratio of flow depth to grain diameter were presented to determine the flow conditions corresponding to the initiation of motion of solitary particles of given shapes. The experiments have revealed that critical flow conditions are dependent not only on the particle size and shape but also on the ratio of flow depth to grain diameter.