  This paper concerns the validity of the Prandtl boundary layer theory in the
inviscid limit for steady incompressible Navier-Stokes flows. The stationary
flows, with small viscosity, are considered on $[0,L]\times \mathbb{R}_{+}$,
assuming a no-slip boundary condition over a moving plate at $y=0$. We
establish the validity of the Prandtl boundary layer expansion and its error
estimates.
