Abstract: Multimode vibration of transducer and its effect on bonding quality was investigated.The velocities of different characteristic marks in the horn and bonding tool were acquired up by Laser Doppler Vibrometer.The experimental results showed that the vibration of transducer is the coupling result of different modes.The velocity ratio of non-dominant mode to dominant mode was gained.The longitudinal flexural vibration is the main non-dominant mode.This kind of longitudinal flexural vibration would "flap" the chip, influencing the bonding area, bonding strength and parallel between gold bumps and substrate.The origin of multimode in transducer was analyzed by FEM model, and some suggestions about restraining the non-dominant mode was brought forward.