Experimental evidence has shown that the natural frequencies of a high-speed spindle decrease with increasing rotational speeds. While this phenomenon is important to the design and operation of high-speed machine tools, it has not been thoroughly analysed. This paper presents a spindle model that can be used to analyse the changes in bearing stirness and the natural frequencies of the system. It is shown that the system dynamics can vary substantially due to bearing centrifugal force and shaft gyroscopic effects. TheJindings can be utilized in optimizing the design and control of high-speed spindle systems.