Abstract: The dual-scale flow and the air entrapment in the preform which cause the meso and micro bubbles formation during liquid composite molding (LCM) process were analyzed. On the basis of volume of fluid (VOF) method, the mathematical model of the resin air two-phase flow in a unit cell was established, and the porous medium resistance and capillary force source term in the model were determined. The numerical simulation method of the above-mentioned mathematical model was implemented based on the UDF function of Fluent software, and the dual-scale flow and bubble formation of the plain woven unit cell was simulated. According to the simulation results on the Rovcloth 2454 fabric, capillary number has a critical role in the bubble formation, the critical capillary Ca^c, at which the micro and meso bubble content is minimal, is exist. When the Ca is lower than Ca^c, meso bubbles are formed. At Ca higher than Ca^c, the micro bubbles are favored easily. The results were compared with the experimental data, and the feasibility of the simulation method was verified.