Microplastic parts are usually fabricated by microinjection molding (mu IM) which is an effective and low cost method. But the defects, such as short shot, often appear during fabricating plastic parts with high aspect ratio structures or complex shapes. a longitudinal ultrasonic- assisted microinjection molding (LU mu IM) method effectively improve the molding quality. In the paper, the mechanism that the ultrasonic vibration impacts on the polymer melt is investigated. Considering from the point view of energy effect, mechanical energy transmission, and mechanical energy conversion, which are divided from the energy of ultrasonic vibration, are analyzed. The model of energy transmission and a new rheological equation including the parameters of ultrasonic vibration are established to describe the rheological behavior of polymer melt in microcavity. The simulation results show that the ultrasonic vibration improves the viscosity field and the velocity field in complex shaped microcavity, and leads to a better filling capability and uniformity of the polymer melt. This research achievement can be used to guide the process flow and parameter selection of LUmIM. (C) 2016 Society of Plastics Engineers