An adaptive actuator failure compensation scheme is developed for attitude tracking control of a spacecraft with unknown inertia parameters. The proposed scheme employs a composite parameter adaptation design which incorporates an adaptive backstepping feedback control law and an adaptive feedforward actuator failure compensator and guarantees the overall closed-loop system stability and asymptotic tracking. A new feature of this adaptive control scheme is the adaptation of the failure pattern parameter estimates, together with that of the failure signal parameter estimates, for direct adaptive actuator failure compensation. Illustrative simulation results of an application to a spacecraft model show that the desired properties are achieved in the presence of the unknown inertia matrix and uncertain actuator failures.