The controller for the power assist robot that is in constant contact with the user requires to be sensitively controlled as per the user"s intention and maintain control stability to ensure the user"s safety. Admittance control is generally used for human intention-based force control. By setting the parameters of the admittance control at a low level it is possible to sensitively control according to the user"s intention. However, too sensitive settings make the system unstable. Therefore, it is difficult to set a fixed admittance parameter in a power assist robot in which dynamics change with an increase in load mass. Consequently, we propose a variable admittance control strategy according to the load mass. The proposed method responds sensitively to the user"s intention by setting the admittance parameters at low during no-load action and ensures stability by setting the admittance parameter high when transmitting high loads. In simulation with a carrying load of 30 kg, the proposed method requires half the interaction force compared with a fixed admittance control when decelerating and has twice faster settling time when stopped. In addition, through experimental verifications, the variable admittance control was proven to reduce the user"s load by 70% compared to load mass.