This paper deals with the design of a 1DOF power assist robot for lifting objects. Human's vertical lifting force, comprises of inertial force and gravitational force, was considered as the desired dynamics of the robot system. We hypothesized that human's perception of object weight due to inertial force might be different from the perceived weight due to gravitational force for lifting object with the power assist robot system. We then simulated the system using MATLAB/SIMULINK and lifted different sizes of objects with the system. We analyzed human's perceived weights as well as load force characteristics for lifting objects with the system. The analyses show that human's perception of object weight is related to load force rates instead of load force magnitudes. The analyses clarify that the magnitude of the peak load force is to be optimized in order to optimize the motion of the power assist robot. On the other hand, load force rate is to be optimized in order to optimize the feeling of heaviness of object manipulated with the power assist robot. The results also introduce the technique of simultaneous optimization of motion and perceived heaviness. Finally, we proposed using the findings to design human-friendly power assist robots for carrying heavy objects in various industries.