When the Roots pump body gas molecules in the gap in the flow of molecular flow. The rotor gap conductance of the Roots pump can be calculated using the Knudsen duct flow guide formula. When Roots pump suction pressure of 133.3 ~ 1.333Pa, the gas molecules in the gap in the flow of molecular flow. The conductance of the gap can be calculated using the Knudsen duct flow guide formula. For the calculation of the interstitial conductance CB, it is assumed that the gas passes through two gap heights σPK, one gap height σPP and two end gap heights σTM and σTNN, and a gap length D + 2a. The gap between the rotor and the selected material, that is, different thermal expansion coefficient, the material of the rotor and the rotor peripheral speed. Such as aluminum alloy rotor circumferential speed u = 30-80m / s, steel rotor u2 = 50-100m / s, while the titanium alloy rotor u2 = 80-150m / s, but also cast iron rotor, u2 lower.