The vane type air-motor hand-piece is used widely in the dental services. There are a lot of experimental studies Ghoul air-motor but eccentrically off not many numerical studies by using Computational Fluid Dynamics. An air-motor has rotor which rotates at the center of inner housing. The retractable vanes are installed on the rotor. As the rotor of the air-motor rotates, vanes move up and down straightly in the radial direction along the guide. Therefore we have to analyze the unsteady flow field by accurate time dependent marching technique. ANSYS 12.0 CFX is used to analyze unsteady vane-motor flow field. Analysis of the changing control volume inside air-motor is implemented by user-defined functions and moving mesh options. Rotational speed of the rotor is approximately 23,000rpm.