Wetting of glass fibers was visualized using an oil with the same refractive index. With both a video-enhanced microscope and a high-magnification video camera we followed the flow front and qualitatively measured the entrapped air bubbles by image analysis. Due to different permeabilities between the fiber mats and bundles, air bubbles are entrapped. Two major kinds of air bubbles are observed : small cylindrical micro voids between fiber filaments (inside fiber bundles) and spherical macro voids outside fiber bundles. These air bubbles can be described by three major mechanisms : initial liquid bypassing/air trapping, later capillary invasion of disordered fiber bundles and air bubble mobilization. Both random and unidirectionaljciber mats were used in this study. Vacuum as well as different fluid viscosities and surface tensions were also investigated which led us to guidelines of optimum processing parameters of fiber wetting for resin transfer molding and structural reaction injection molding They are low viscosity vacuum, high mold temperature, and high pressure.