This paper describes the Lyapunov-based force control suppressing the coupled bending and torsional vibrations of a one-link flexible arm with a rigid tip body. On the basis of the distributed parameter model, the output feedback control law is constructed using Lyapunov method, and the asymptotic stability of the closed-loop system is proved. The proposed controller consists of the PD feedback of the motor angle and a feedback of the bending strain at the root of the flexible arm. Some simulations are performed to show the effectiveness of the proposed controller.