Turbomolecular pumps (TMP) have been widely used in low-pressure processing, because TMP can pump a large flow of gas as well as can exhaust the system to a suitable base pressure. A TMP, however, has the problem that its pumping speed is severely degraded at inlet pressures higher than similar to 10 mTorr, where most of the low-pressure processing of semiconductor manufacturing is performed. In this study, it is confirmed that the occurrence of such a degradation in TMP performance is primarily determined by the inlet pressure of TMP and not by the outlet pressure. As the inlet pressure is increased to similar to 10 mTorr, the gas flow in the front blades as well as the oar blades of the TMP goes into the transition flow, and TMP pumping speed consequently begins to decrease. Once the degradation occurs, however, the pumping speed is strongly dependent upon the pumping speed of the backing pumps, namely the foreline pressure. In order to improve the TMP performance under high gas flow, a characteristic dimension in the pump, particularly at the entrance, such as the distance between each blade vane and the distance between the rotor blade and the stator blade, was shortened to increase Knudsen's number. This enables us to maintain the molecular flow region at higher pressures and to improve the TMP performance under high gas flow. The newly developed TMP can endure a gas flow as great as similar to 1500 seem with maintaining the acceptable pumping speed. We have also experimentally measured the impurity level in the chamber due to back-diffusion through TMP, as well as the impurity compression ratio under high gas flow. It is demonstrated that the impurity level in the chamber can be made several orders of magnitude lower than that for the best performance of TMP under ultrahigh vacuum operation when adequate gas flow is fed into the chamber.