Higher robot arm speeds are usually accompanied by undesirable vibration. One of the major reasons for this is the considerable flexibility in harmonic drives. State feedback control is known to be an effective means of suppressing such vibrations. With this control method, however, it is usually difficult to determine an optimum feedback gain. This paper describes a method for controlling robot arm vibration using the virtual damping system. By using software, this method increases the viscous friction of the robot arm. With the virtual damping system, feedback gain is easily determined based on a method that assimilates information about the robot arm's jumping, acceleration and velocity at a constant ration. This information is then calculated by an observer. Experimental results show that this method is effective in practical applications.