Abstract: In order to investigate the effects of ultrasonic vibration on injection molding characteristics of fiber reinforce composites, by using a self-developed ultrasonic assisted injection molding visualization experimental device, visualization experiments under ultrasonic field effects were conducted on glass fiber (GF) reinforced polypropylene (PP) composites with different GF contents. Effects of ultrasonic power on filling flow behaviors of composite melts were observed and analyzed. In addition, the effects of ultrasonic power on fiber orientation of composites were also analyzed by metallographic observation of different parts of specimens. The results show that ultrasonic power affects the composites filling flow behaviors of injection modeling and the fiber orientation of products. While the fiber content of composites also has direct influences on the effects of ultrasonic vibration. When the fiber content is lower, the main reason which influences the filling flow ability of composites and fiber orientation is the effect of ultrasonic vibration on micromorphology of matrix materials; when the fiber content is higher, the main reason which influences the filling flow ability of composites and fiber orientation is the effect of ultrasonic vibration on fibers. The investigation results provide the basis for the development of ultrasonic assistant molding technology for composites.