Abstract: By coupling of elastic motion, we mean such a process, in which the forces resulting from elastic vibration act on the same vehicle, and lead to new elastic motions. It is the coupling that maintains vibration.When the equations of motion are rewritten in matrix, the coupling factor is a bilinear form in modal functions or their derivatives determined at the impelling point and impelled point respectivelly, with T-1 M as the coefficient of the form.Aerodynamic force is the most essential and it may give rise to coupling between the distinct vibrations. When the engine is installed on a flexible body, the propulsive force also results in vibrating. In the feedback control system, control force will produce vibrating that . is related to the points of measurement and control. But if an open loop control is used, no coupling exists.At the end,coupling factors are applied to. optimizing the positions of engine sensor and actuator.Practical examples are given.