Abstract A new structure for reducing the windage power loss of a high-speed rotor using a spiral grooved viscous vacuum pump combined with an aerodynamic step thrust bearing is proposed. The proposed structure can pump out the air from within the sealed space of the motor housing by using the pumping effect of the spiral grooves and thereby reduce the windage power loss of the rotor. In addition, a small gap was automatically formed between the rotor and the viscous vacuum pump by using the force balance between the aerodynamic step thrust bearing and the elastic material supporting the flat plate of the vacuum pump. It was numerically shown that the proposed structure could reduce the pressure in the sealed space of the motor housing to 0.02 MPa at 30,000 rpm at a gap of 10 μm. In addition, the calculated results at 10,000 rpm were compared with the experimental results and showed good agreement and the proposed structure was very useful in reducing the windage power loss of a high-speed motor.