In this study, wicking in flax fabrics was investigated through an experimental method coupled with Washburn theory. This method was previously shown effective for carbon fabrics (Pucci et al. [21]) to determine morphological characteristics and apparent advancing contact angles. For natural fibers, Washburn equation is not sufficient to describe wicking because of moisture sorption that causes fiber swelling during liquid imbibition. Some flax fabrics were submitted to a thermal treatment known to modify fibers chemistry. Wicking tests with water were performed on both fabrics at different fiber volume ratios. It was observed that wicking trend is very different for these two types of reinforcements: treated fabrics show typical linear trends described by Washburn equation, while untreated flax fabrics lose linearity during wicking. Sorption tests performed on elementary fibers proved that swelling is less significant for treated flax fibers. A model was proposed and was shown to describe properly wicking in natural fabrics undergoing swelling.