Tow bundles inside a quasi-unidirectional non-crimp fabric are maintained by sewing threads that induce variation in the bundles’ shape. Indeed, the sewing threads apply a light clamping force that gives a periodical and sinusoidal shape to the tow’s cross-sectional area. This tow cross-sectional area heterogeneity, as a function of position in the fabric, induces a variation of the permeability ... [Show full abstract] values. Consequently, while injecting liquid into the fibrous bed, preform's impregnation is influenced as well as the fabric’s void content. The aim of this paper is to consider the effect of tow cross-sectional area heterogeneity on the quality of the manufactured composite part. It leads to reveal the influence of the fibrous reinforcement’s microstructure on the air bubble creation and compression phenomena, especially in terms of process time and the micro and macro air bubble distribution.