System identification experiments have been performed as one of the on-orbit experiments of ETS-VI which was launched in August, 1994. There are two types of identification experiments: impulse excitation and random excitation. The method and the results of random excitation are summarized in this paper. In the random excitation experiments, thruster injection direction is changed randomly according to the M-sequence. Using the on-orbit data, parameter estimation by the ARX model is executed in order to construct a mathematical model of ETS-VI. The behavior of the mathematical model is compared with that of the on-orbit data in both the frequency and the time domains. The results show that the mathematical model accurately reproduces the on-orbit behavior of ETS-VI.