The limiting load capacity of air foil thrust bearings at extremely high operating speeds is theoretically investigated. The limiting load capacity of a sector is shown to increase as the angular extent ${eta}$ and the inlet film thickness $h_1$ of the bearing increase, while it decreases with an increase in the ramp ratio b and the compliance ${alpha}$ of the bearing. But it is found that the angular extent of the bearing is not related to the total limiting load capacity of the $360^{circ}$ thrust bearing.