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In this paper the principles of simultaneous measurement of three orthogonal force vectors Fx, Fy, Fz and three orthogonal torque vectors Mx, My, Mz to design a six axis force/torque sensor are considered. At first, a new index (η) for a qualitative comparison of six-axis force/torque sensors is proposed and then, cross-coupling error of several sensors presented in previous studies is evaluated and compared by using the new index. In the following, a systematic method for designing the six-axis force/torque sensor is described using numerical optimization procedure. This method is based on interactive interface between the SQP algorithm created in MATLAB and FEM analysis in ANSYS software. The geometry of sensor structure is selected to be a modified Maltese cross type. Principle cross-coupling error is chosen as the objective function to optimize four geometrical design variables of the sensor structure. Also, strain gauge sensitivity, maximum applied stress and geometric sizes of the sensor structure as constraints are formulated in problem. Results show that principle cross-coupling error of the optimal sensor design is less than 1.49% with a high moment to force specification (0.1 N.m/N).