Abstract: The nylon nonwoven fabric (Polyamide Nonwoven Fabric, PNF) was selected as structural toughening layer to investigate its effect on the permeability of fiber preforms during resin transfer molding (RTM) process. Results show that under unsaturated radial flow pattern, the permeability of the interlaminar toughened preform along the direction of fiber (5.2×10^-12 m²) is lower than that of the untoughened preform (7.1×10^-12 m²). On the contrary, the permeability of the toughened preform perpendicular to the fiber direction (2.3×10^-12 m²) is higher than that of the untoughened preform (1.6×10^-12 m²). Moreover, the unidirectional saturated flow permeability of interlaminar toughened preform is 2.6×10^-12 m², which is much lower than that of untoughened preform (1.9×10^-11 m²), decreased by an order of magnitude. The z -directional saturated permeability also dramatically decreases from 1.3×10^-13 m² (untoughened preform) to 2.5×10^-14 m², decreased by an order of magnitude. The interlaminar morphology analysis results reveal that the decrease of the permeability of preform mainly results from the inhibition of rapid flow of resin, introduced by PNF as structural layers and the increment of interlaminar fiber contents rising from 55.3vol% to 63.7vol%.