A high-speed spindle generates large heat through friction at contact areas, thereby causing thermal expansion, which adversely affects machining accuracy. The objective of this work was to simulate the temperature increase and the thermal deformation of an oil/air lubricated spindle. A finite element method (FEM) was used to analyze the thermal behavior of the spindle. Heat conductivity measurements and inverse analysis were conducted to estimate the conductivity coefficients of ball bearings; then, the temperature increase and the thermal expansion were computed by numerical simulation. Previously measured data were compared with numerical simulation results.