The control issue of mechanical systems with mai-machine interactions is discussed in this paper. How to cortrol the interactions among the system, its environment an the human operator, so that the desired interaction is reaized, is the key issue for the control of such a mechanical sysem. A control algorithm for the mechanical system with mai-machine interaction such as a robot system for manrob t cooperation, master-slave manipulator system, powerste4ring system etc. is proposed in this paper. We first mo1el the mechanical system, which has interactions with its environment and an operator, then propose a control algrithm so that the desired interaction is realized. The proposed control system consists of two controllers; one genfrates the desired motion of the system based on the force appied to the mechanical system, and the other one controls the nteraction between the system and its environment. The projiosed control algorithm specifies both force augmentation and maneuverability of the system. The stability of the sys em is also discussed. The algorithm is applied to a pla4ar manipulator with one degree of freedom.The experimental system illustrates the concept of the system.