Abstract: The properties of high speed airflow lubrication in micro-clearance of spiral-groove face seals were studied with consideration of entrance pressure loss and choked flow effect. The seal parameters such as opening force, leakage and gas film stiffness were numerically calculated and discussed under different conditions of clearance, seal pressure and rotational speed. The results show that the choked flow effect made the exit pressure higher than the ambient pressure, while the entrance pressure loss led to inlet pressure drop. These resulted in significant decrease of the leakage rate and the gas film stiffness, but increase of the opening force. The choked flow effect increased with increasing seal pressure and seal clearance, which led to obvious decrease of the leakage rate and the gas film stiffness. When the seal pressure reached up to 10MPa, the leakage rate decreased 20%, meanwhile the gas film stiffness decreased more than 30%.