The monomer conversion efficiency and average particle size and its distribution of vinyl acetate (VAC) polymerization by conventional and ultrasonic-assisted means individually were compared. With adding ultrasonic wave in the VAC polymerization, the monomer conversion efficiency was increased from 92.5 % to 98.8%, and the average particle size of latex was reduced from 136.7nm to 101.4nm. During the process of ultrasonic-assisted polymerization of VAC system, the effects of initiator concentration and reaction temperature on the conversion efficiency and average particle size distribution were studied. The experimental results demonstrated that the radicals generated from ultrasonic energy solely was not enough to propel the polymerization perfectly and the radicals produced by decomposition of initiator play an important role of driving polymerization. Furthermore, the mechanism of ultrasonic-assisted emulsion polymerization was briefly deduced. It also found that ultrasonic wave maintained a constant supply of radicals and these radicals were able to reactivate some of the non-active polymer chains, so that it resulted in the higher conversion efficiency of monomer.