This paper studies the problem of periodic stabilization of nonlinear underactuated mechanical systems. In opposition to the problem of stabilization of underactuated systems (i.e. acrobots, pendubots, etc.) to a fixed equilibrium, the problem of orbital stabilization of underactuated systems consists in finding control that leads to a stable periodic orbits. The problem is relevant to a class of mechanical systems aimed at operating under periodic motion (orbits), i.e. walking mechanisms.