Abstract: Multi-layer metal gate technology is required to develop the high speed and low power consumption CMOS circuit. For the bi-layer metal gate with different thickness,the difference of work function between the semiconductor and metal gate stacks lies on the undermost metal gate,which offers a theory foundation to alter the CMOS threshold voltage by only modulation work function of the undermost metal gate. When the thickness of undermost metal gate is less than that of its maximal dipole layer,the work function is more than that of thick film,and the effect of tunable work function is obtained by analyzing the energy band of bi-layer metal.