This paper investigates the vibration problems associated with the attachment of flexible disks to high speed, flexible rotors. A semi-analytical finite element method has been employed to model two flexible disks attached to a flexible rotor supported by three ball bearings. A two-nodes beam element of 8-DOF is used to model the flexural vibrations of the flexible rotor while a four-node (arranged on two perpendicular diameters) annular circular plate element of 8-DOF is used to model the transverse vibrations of the flexible disks. The system natural frequencies and critical speeds are computed in a speed rang up to a 5000 RPM. An experimental test rig has been employed to validate the theoretical findings. An acceptable matching has been found between the experimental and the simulation results. Most importantly, the results reveal that the rotor natural frequencies and critical speeds are marginally influenced by the disk flexibility. The main contribution of this work is that the inclusion of disk flexibility in the modeling process effectively reduces the modeling error, which seems to be crucial for some applications.