This is a continuation of the paper [15] on nonlinear boundary layers of the Boltzmann equation where the existence is established and shown to be strongly dependent on the Mach number <i>M</i> <sup> <i></i> </sup> of the Maxwellian state at far field. In this paper, when <i>M</i> <sup> <i></i> </sup><1, we will show that the linearized operator has the exponential decay in time property and therefore a bootstrapping argument yields nonlinear stability of the boundary layers.