To solve the trajectory tracking problem of a quadrotor UAV, an optimal iterative learning controller based on the disturbance observer is designed.A discrete kinematics model of the quadrotor UAV is established in consideration of external disturbances.A discrete disturbance observer is designed to estimate the unknown disturbance, and the convergence of disturbance estimation errors is strictly proved by using the Lyapunov method.On the basis of compensating for the disturbance, an optimal iterative learning controller is designed to ensure the asymptotic convergence of the tracking error of the quadrotor UAV to the desired trajectory.Finally, the effectiveness of the disturbance observer and the iterative learning controller is verified by simulation.