Fiber reinforced composite material and magnetic powder containing epoxy composite were employed for the materials of the spindle shaft and squirrel cage rotor. respectively to enhance the dynamic performance of high speed built-in type air spindle system because the bending natural frequency of spindle system increases when the specific bending stiffness (EI/rho) of the shaft is high and rotor mass is reduced. Although polymer based composite rotors have low eddy current characteristics, the mechanical and magnetic properties of the composite rotor at elevated temperatures may be degraded due to the heat transfer from stator coils. Therefore, in this paper. the mechanical and magnetic material properties of powder containing composites such as storage modulus (E), coefficient of thermal expansion (alpha), thermal Conductivity (k) and magnetization curves (B-H curves) were measured with respect to temperature and magnetic powder content. Then the thermal and magnetic analysers were performed with appropriate boundary and loading conditions, and the optimal design conditions of squirrel cage rotor were proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.