This paper introduces a sensor-less grasping control technique for two finger robot hands. Each finger of the prototype of hands consists of two links driven by electrical motors and mechanical spring is mounted on each joint. That is the fingers can move and realize intended poses against the force of springs. But it causes the necessary condition of observability and we can design an observer in order to estimate the joint angles of fingers. The basic principle and a few brief applications of the sensor-less control technique was already presented by Shimada et.al, but our reserch is extended to practical robot hand systems in this paper. Although rotary encoders are mounted on all of joints, they are used not to control but only to detect the joint angles in order to evaluate the presented sensor-less control technique.