Abstract： A synthesized adaptive and sliding-mode observer (SMO) was designed for the incipient fault in a class of linear system with unknown disturbance, which can not only estimate the incipient fault, but also have strong robustness against unknown disturbances. The linear system was transformed by coordinate transformation into two subsystems, one of which was free from disturbances and an adaptive observer is designed to estimate the incipient fault, the other of which was affected by disturbances and incipient faults and a sliding mode observer is designed to eliminate the effect of unknown disturbances and ensure the robustness of the system. Simulation results of the quadrotor helicopter verify the efficiency of the proposed adaptive and sliding-mode observer-based incipient fault detection algorithm.