Ultrasonic technique is proved to own higher efficiency than the traditional methods such as electroosmosis and electrohydrodynamic in driving and controlling fluid motion in microfluidic devices. However, few studies are done for theoretically studying the fluid motion induced by the acoustic field due to the complexity of the acoustic field and its induced flow field. In this paper, we present a theoretical study of fluid motion acoustically induced in microchannel. Navier-Stokes equations are first employed for 1-D analytical model for acoustic radiation pressure and acoustic streaming in the microchannel where a traveling wave or standing wave exists. Then, finite element method in ANSYS is used to analyze the transient fluid field and the results of the first-order acoustic velocity field in microchannel are obtained, and furthermore, the distribution of acoustic streaming and driving force in microchannel are calculated and discussed.