In this paper, an actuator fault detection and isolation scheme for a class of nonlinear systems with uncertainty is considered. The uncertainty is allowed to have a nonlinear bound which is a general function of the state variables. A sliding mode observer is established first based on a constrained Lyapunov equation. Then, the equivalent output error injection signal is employed to reconstruct the fault signal using the characteristics of the sliding mode observer and the structure of the uncertainty. Finally, a simulation study of the HIRM aircraft system is presented to show the effectiveness of the scheme.