For the problems of actuator fault detection based on interval observers for a class of nonlinear systems subjected to the unknown nonlinear function (including the system uncertainty, the external disturbance, etc). Firstly, for the system without actuator fault, based on the known boundary information of the unknown nonlinear term, two interval observers are developed. Then, the residual errors that can be used for detecting the actuator fault are constructed by using the outputs of interval observers and those of systems. Thus, the problems of actuator fault detection based on interval observers are resolved. Finally, two numerical simulation examples are given to illustrate the correctness and the effectiveness of the proposed methods.