Abstract: For the micro torsion bar used in inertial navigation, significant errors are often induced in the torsion rigidity calculation using the twist theorem of prismatic bars, because the transitional fillet of the revolving generatrix can not be neglected. Based on the twist theorem of bars with variant section, this paper deduces the analytical expression for the torsion rigidity of the micro torsion bar with the transitional fillet. Then, the influence of the fillet dimension on the calculated result is investigated. Finally, the accuracy and validity of the deduced expression is verified by a numerical example. It shows that the error drops to less than 0.1% from 17% considering the fillet’s effect.