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In this paper, we designed and controlled a 1 DOF power assist robot for lifting objects based on human operator's unimanual and bimanual weight discrimination. At first, we established psychophysical relationships between the actual weights and the power assisted weights for the objects lifted with the power assist robot. We also determined the excess of the load forces that humans applied while lifting objects with the robot. In everyday life, human decides to employ one or two hands to manipulate objects on the basis of object's physical characteristics. Hence, the psychophysical relationships were established and the excess of the load forces were determined for two protocols independently. The protocols were: 1) unimanual lift, and 2) bimanual lift. Then, we modified the control system of the power assist robot based on the psychophysical relationships and the load force characteristics. The modified control system reduced the excessive load forces applied by humans and thus enhanced maneuverability, operability, naturalness, ease of use, stability, safety etc. of the robot system significantly. Then, we suggested using the findings to design power assist robots for carrying heavy objects in various industries.