Abstract: A new MEMS device that can separate microparticles onchip by the use of bulk-mode excited membrane vibration is designed and microfabricated.The performance of the microdevice is studied in detail,especially the force for separating particles in the microdevice.Starting from the one-dimensional analytical mode of the acoustic radiation force,we study the factors affecting particles separation within the liquid medium.Then,finite element method in ANSYS is used to analyze the vibration modes of the microdevice,the acoustic field and flow field in the microdevice due to the membrane vibration.With these analyses,we obtain the forces acting on different particles in the water,blood plasma and milk medium when the microdevice vibrates at different frequencies.