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An analytical investigation is presented on leakage-flow-induced vibrations of an axisymmetric body. In the case of a narrow annular passage between a tapered outer cylinder and a tapered axisymmetric body, the fluid-dynamic forces acting on the body have been divided into the fluid-dynamic mass, damping and stiffness. The following conclusions were obtained through research on those coefficients. The dynamic instability of the systems is due to fhe phade lag of the fluid-dynamic force for displacement of the body caused by harmonic vibration. The fluid-dynamic damping is obtained in two parts: one is independent of the frequency and the other is a function of the frequency, and the negative damping is caused by the fluid-dynamic damping which is a function of the frequency. When the frequency is very large, the fluid-dynamic mass agrees with added mass within the fluid at rest.