Abstract: A new concept "isotropy" for robot's force sensors is presented on the basis of Fisher's information matrix M. Special emphasis are put on the equivalence representation and its invariance with respect to coordinate transformation. Ideally, the isotropic sensors may be thought of as maximally informative. However, it is necessary to identify the behavior of force sensors in performing robotic assemby tasks. The task models in the wrench space can be used to define the quality measures for assembly planning and other tasks. It is shown that the optimality conditions for assembly execution are related to the tasks, invironment model and the structure parameters of robot's force sensors.