Abstract: The void formation during RTM processing seems to be inevitable and the presence of voids has a deleterious effect on the mechanical properties of textile composites. The purpose of this study is to investigate the mechanism of void formation in multi-layer woven fabrics. Two unit cell models were suggested to represent the cross sections of multi-layer woven fabrics with two typical layering patterns. Based on these two geometrical models, an analytical model was developed to describe the mechanism of void formation during RTM processing. The flow fronts and void formation in these two cells were also numerically simulated using a control-volume method. The simulated results agree well with those predicted by the analytical model. The results show that, for a given fiber preform, the ratio of the weft axial permeability K^a_b to the warp transverse permeability K^t_b is responsible for the size of the void.