Abstract: In this study, an air-coupled ultrasonic nondestructive evaluation (NDE) method was used to measure the resin flow of a partially impregnated prepreg through thickness of fibers layer. The partially impregnated prepregs were put into a vacuum bag, and then assembled on the C-scan sweep mechanism. Vacuum was applied to the specimens and ultrasonic images at various times were obtained by using the air-coupled ultrsound evaluation instrument. Therefore, flow variation of resin penetrating into fibrous webs was monitored in situ by the ultrasonic images. As well, the permeability of the carbon fiber webs was calculated from Darcy’s law. To evaluate the validity of the used NDE method, metallographic specimens corresponding to each C-scan sample were prepared and observed, and the yielded microphotographs demonstrated the rationality of the NDE method. Results show that the proposed air-coupled ultrasonic evaluation method is an effective way for in situ monitoring resin flow through thickness of fibers layers.