Abstract: A novel type of caliber adaptor was developed to match large inlet-diameter of turbo molecular pump and small outlet-diameter of vacuum container. The caliber adapter comprises a conical reflective screen over the flat turbine, a truncated cone with curved wall and large/small diameter flanges. The flow field in the caliber adaptor was mathematically modeled, theoretically analyzed with free molecular flow assumption and in test particle Monte Carlo method, and numerically simulated with Molflow + software. The influence of the adaptor geometry on the transmission probability was investigated for design optimization. The simulated results show that the dip-angle (α) of the truncated cone and the base-angle (β) of the conical reflective screen have a major impact. To be specific, the new caliber adaptor, with α in 60° ~ 70°and β in 25° ~ 45° ranges, theoretically increases the pumping rate of turbo molecular pump by about 5%.