In this paper a nonlinear attitude tracking control scheme is developed for a small-scaled unmanned helicopter under input constraints. Via the analysis of the properties associated with the helicopter's rotor dynamics, the elevator servo input, the aileron servo input, and the rudder servo input are chosen to be the control inputs to be designed. Their constraints in amplitude under hovering flight is taken into account by using the robust bounded terms in the controller design. The asymptotic convergence of the tracking error is guaranteed with the Lyapunov-based stability analysis.