Abstract: The sensing and control of the force or torque of industry robots contacting its environments is one of the focuses of presentday world's robot research. However, robot manipulators in force control mode often become unstable during contact with stiff environments. This is called dynamic instability. Dynamic instability of force control is caused by: 1) the unmodel dynamic of manipulator's servo motors, 2) the unmodel dynamic of force sensors, 3) the stiffness and dynamic of environments, 4) the sampling control delay of computer control. This paper discusses how these causes affect the dynamic stability. Finally the conclusion of simulations and experiments is given.