This paper presents a new trajectory linearization control scheme for a class of nonlinear systems subject to harmonic disturbance. It is supposed that the frequency of the disturbance is known, but the amplitude and the phase axe unknown. A disturbance observer dynamics is constructed to estimate the harmonic disturbance,and then the estimation is used to implement a compensation control law to cancel the disturbance. By Lyapunovs direct method, a rigorous pool shows that the composite error of the closed-loop system can approach zero exponen-tially. Finally, the proposed method is illustrated by the application to control of an inverted pendulum. Compared with two existing methods, the proposed method demonstrates better performance in tracking error and response time.