Abstract: The mean free path and the viscosity of helium-air gas mixtures were calculated as a function of the helium fraction. The second-order-slip modified Reynolds equation was solved using the finite element method. The hydrodynamic pressure and film thickness of the micro-bearing were obtained. The effects of helium fraction, groove depth and angular velocity on the bearing characteristics of the micro-bearing were investigated. The results show that when the groove depth increased from 1 μm to 10 μm, the film thickness of the micro-bearing firstly increased and then decreased, reaching a maximum at the groove depth of 5 μm, which represented an optimum bearing capacity. In addition, at a groove depth less than 5 μm, the mean free path of the gas mixtures had a dominant effect on the film thickness of the micro-bearing. However, when the groove depth was larger than 5 μm, the effect of the gas viscosity dominated.