A method for reducing vibration of a robot arm includes: a step of mounting at least one inertia actuator and at least one vibration signal capturing unit to a processing end of a robot arm; a step of applying the at least one vibration signal capturing unit to detect a vibration generated at the processing end of the robot arm so as to generate a vibration signal; a step of applying a central processing unit to evaluate the vibration signal and coordinates of the processing end of the robot arm so as to capture at least one set of corresponding control parameters for calculating at least one output force; and, a step of having the inertia actuator to apply the output force to the processing end of the robot arm for counteracting the vibration at the processing end of the robot arm.