Acoustofluidics is the study of acoustics in microfluidics and a subset of fluid mechanics in micro-scale. It is the basis for analyzing many of the laboratory applications, including the separation and particle sorting, cleaning and mixing of multiphase systems. In this dissertation, the effect of applying the acoustic field on the particles in the acoustic microchannel and motion of the particles under the influence of these waves are investigated using finite element method. The effect of the microchannel walls on the flow around the particle and its motion variations is examined. By modeling the two particles in the microchannel, the interaction force of the particles in the acoustic field is calculated and analyzed. Using this model, the effect of changing the radius of the particles and the distance between them in the magnitude of this force have been studied. In addition, by adding a third particle to the model, the variation of force on two particles under the influence of this particle with respect to its distance to two particles have been investigated. The proposed model is verified against analytical solution for single particle and two particles in the Rayleigh limit and then the cases are analyzed for particle radii out of this range. The three-particle model also helps studying the force pattern between three particles possible for which no analytical solution is available, and also has the ability to simulate the motion of multi-particle model. Keywords : Acoustic waves, Microchannel, Finite element method, Acoustic radiation force, Acoustic interaction force