The objective of the present paper is to clarify the heat partition ratio in spindle units with aerostatic bearings.Heat generated in the journal bearing clearance is transferred to the spindle, bush/housing and exhaust air.In order to develop the thermal model on the basis of experimental evidences, a test machine was used running to a maximum rotational speed of 20000 min' with a 60 mm diameter spindle supported by aerostatic journal bearings of 20,um radial clearance, and the temperatures were measured of the housing, bush and the interface between the bush and air film as well as the inlet and outlet air.A finite element model has been developed to evaluate the partition ratio.The analysis shows that the ratios of the heat flowing into the spindle, into the bush and into the exhaust air to the generated heat are about 1/3 respectively in the test air spindle unit, and that increasing the bearing clearance is most effective to reduce the air film temperature.