Abstract: The Shields curve is often used to judge the criterion for incipient sediment motion. The curve for all the flow regimes is derived on the basis of the boundary layer theory. The Shields parameter and curve are modified in order to take into account the effect of viscosity. The modified curve is then re-explained from the viewpoint of boundary layer. Results show that the curve's shape exhibits a linear distribution in the smooth turbulent and laminar regime. In the transition regime, the curve is similar to that representing the friction law for steady flow, and finally becomes a horizontal curve in the rough turbulent regime. The modified Shields curve keeps the consistent shape with the original one. The modified curve is applicable to predict the incipient motion of both fine and coarse sediment particles. The prediction agrees well with measured data. In fact, the Shields curve represents the relationship between the threshold bed shear stress and the flow status around the particle. The threshold bed shear stress of one particle is different under different flow regime conditions.