This paper investigates non-synchronous vibration (NSV) mechanism of a high-speed axial compressor with three different rotor tip clearances. Numerical simulations for 1=7th annulus periodic sector are performed using an unsteady Reynolds-averaged Navier-Stokes(URANS) solver with a fully conservative sliding boundary condition to cap- ture wake unsteadiness between the rotor and stator blades. The simulated NSV shows that the frequency and amplitude are strongly influenced by the tip clearance size and shape. The predicted NSV frequency is in good agreement with the experiment. The maximum amplitude of the NSV occurs at about 78% span of the rotor suction leading edge regardless of tip clearance due to a strong interaction of incoming flow, tip leakage flow and tip vortex. The instability of tornado like tip vortex oscillating in streamwise direction appears to be the main cause of the NSV observed in this study.