In this study, an ultrasound visualization system has been set up for in-situ monitoring of the resin flow impregnating through opaque carbon fiber reinforcements during the vacuum-assisted resin transfer molding. The flow front advancement through the carbon fabrics covered by a bagging film can be identified and visualized by the high frequency B (brightness) mode ultrasound imaging technique. The resin advancement in the opaque carbon fabrics has turned out to form a non-uniform plug flow along the pressure gradient direction and the potential void formation can be observed from the mesoscopic resin flow behavior: the inter-tow regions have been preferentially filled by the resin fluid and the fiber tow region behaves as a sink that probably remain as a void defect. The local unsaturated transient velocity of the resin flow has been also evaluated, which is particularly important in understanding saturation behaviors in dual-scale fabrics and is hardly measurable by other means. In spite of the hardware limitations on the resolution, the proposed ultrasound visualization system can provide a less expensive and portable visualization tool to understand the microstructure of opaque reinforcements and monitor the resin flow behavior during the vacuum-assisted resin transfer molding process in the industrial composite manufacturing environment.