The in this paper described torsion force measurement system contributes to the Lorentz force velocimetry application, where the horizontal force in combination with dead load of 1 kg is measured. The theoretically calculated stiffness of the system is 0.5 N/m, undamped nature frequency is 0.06 Hz and it is expected to achieve the force resolution of 0.5 nN. Initially the electrical voltage as the output signal of the photoelectrical position sensor is converted into angle with an autocollimator system. Then the output signal of the position sensor is calibrated into force with the help of a known tilt force generated by tilt angle and a normal mass piece, the convert factors for the two positon sensors are 37.1038 µN/V and 38.7247 µN/V. A standard deviation of 5.5 nN over one hour has also been achieved and the linear working range of the system is nearly ± 40 µN.