The relation between balance characteristics and its stiffness is discussed, emphasizing the evaluation of first and second order interference terms of the balance. Three kinds of internal balances are used for this purpose. Balance characteristics are determined with the automatic balance calibration system of the NAL-TWT (Transonic Wind Tunnel). During balance calibration, the strokes of six electric actuators for repositioning are stored as a data file. The balance deflections like capital delta X, capital delta Y, capital delta Z, capital delta phi, capital delta theta and capital delta psi at full loading are calculated with this data file. They are called 'balance stiffness' in this paper. The relation between balance characteristics and the stiffness is discussed. In case of high stiffness (small deflection), the interference terms in the balance characteristics are small in comparison with the low stiffness balances.