For practical reasons, past studies on vibration control of a cantilever beam with a piezoelectric actuator have been devoted to reducing the free vibration in which the first mode is dominant. In the case of forced vibration, higher modes must be actively damped. This paper describes a control method for improving modal damping ratios of a cantilever beam up to the third mode using a piezoelectric actuator. First, an inverse system is designed to provide estimators of the entire state. Next, the control law, which is based on the feedback of linear combination of modal velocity, is discussed. Finally, an experiment on free and forced vibration is carried out. From calculated and experimental results, we find that modal damping ratios of a cantilever beam are improved by use of the above control method.