Abstract: A new regenerative chatter stability prediction model is presented for milling process with internally damped rotating tapered composite cutter bar. Based on the viscoelastic constitutive relation of composite combined with Hamilton principle and Rayleigh beam theory, the chatter governing equations of system are obtained in fixed and rotating coordinate frame, respectively. Modal damping loss factors of composite bar are calculated by free vibration analysis and energy approach. The critical rotating speed and instability threshold are evaluated by solving the characteristic problem of the rotating composite cutter bar. The stability of the system is predicted using both zero order approximation frequency domain method and semi-discrete time domain method. It is shown that both rotating and fixed frame based models give the same results for dynamically symmetric tools. For the milling process with rotating composite cutter bar, it can be found that material internal damping can lead to a new chatter instability zone at high rotational speeds. This new chatter staring point equals to the instability threshold of rotating composite cutter bar.