Angular contact ball bearings have been widely used in machine tool spindles, and the bearing preload plays an important role on the performance of the spindle.With the development of high speed machining, especially for high speed milling, heavy cutting at a low speed and light cutting at a high speed are often performed on a single machine tool spindle, thus, high stiffness at low speed and low temperature rise at high speed are required.The traditional constant pressure preload method cannot meet the technical requirement of this kind of spindle any more.The variable preload technology is systematically investigated in this paper.At high speed range, FEM method is used to analyze the temperature distribution of the spindle, and the variable spindle preload is determined according to the constraint of temperature rise of bearings.At low speed range, the spindle preload is resolved by the fatigue life of bearings.The dynamic stiffness of the variable preload spindle is analyzed utilizing the Transfer Matrix Method (TMM) and a nonlinear bearing model that includes the centrifugal force and gyroscopic effects.An experimental set-up for the variable preload spindle is developed using hydraulic pressure that can automatically adjust the bearing preload.The proposed method to determine variable preload is verified experimentally by measuring the dynamic stiffness of the spindle and the temperature rise of the test bearing.The results show that the variable preload spindle gives outstanding behavior that the temperature rise at high speed is lower than that of the constant pressure preload spindle, and the dynamic stiffness at low speed range is significantly increased.