Low-cost but high performance robot arms are required for widespread use of service robots. Most robot arms use expensive motors and speed reducers to provide torques sufficient to support the robot mass and payload. If the gravitational torques due to the robot mass, which is usually much greater than the payload, can be compensated by some means, the robot would need much smaller torques, which can be delivered by cheap actuator modules. To this end, we propose a counterbalance mechanism which can completely counterbalance the gravitational torques due to the robot mass. Since most 6 DOF robots have 3 pitch joints, which are subject to gravitational torques due to the robot mass, we propose a 3 DOF counterbalance mechanism based on the double parallelogram mechanism, in which reference planes are provided to each joint for proper counterbalancing. A 5 DOF counterbalance robot arm was built to demonstrate the performance of the proposed mechanism. Simulation and experimental results showed that the proposed mechanism had effectively decreased the torque required to support the robot mass, thus allowing the prospective use of low-cost motors and speed reducers for high performance root arms.