The purpose of this study was to compare the flying route of small tale tennis balls with that of large ones under different initial velocity and spin speed. The research method was to make analysis by the laws of Fluid mechanics and Numerical methods. The results showed that 1. the coefficients of restitution were 0.875 and 0.867 respectively for small and large balls (those with the diameter under 38mm are called small balls, while those with the diameter 40 mm are large balls), 2. Supposing the balls were hit with the spin speed 50 rev/s, and the velocity increased from 0 m/sec to 20 m/sec, then both their Drag force and Maguns force would increase with the speed. However, when the velocity ranged from 15 m/sec to 20 m/sec, the increasing rate of the Maguns force curve would decline. Another finding is that large balls endured bigger Drag force and Maguns force than small ones, 3. Whether the balls were top-spinning, no-spinning, or bottom-spinning, with the same and height, the flying distance of small balls was longer than that of large ones.