Abstract: Edge effect is one of the common draping defect s of a fiber preform in liquid composite molding (LCM)processes. The unidirectional flow method was adopted to study the influences of the edge effect on the preform permeability and the mold filling. Based on the theoretic model of equivalent permeability , the edge effect s under theconditions of different fiber content s and gap widths were simulated and analyzed. A characterization factor for thest rength of the edge effect was also proposed. Furthermore , the on-line strategic cont rol and measurement for edgeeffect s were described taking the mold filling in a complex mold cavity for a case.