For the use of natural fibers in composite materials it is often necessary to improve the compatibility between fiber (sizing) and polymer matrix systems, e.g., by increasing the number of functional groups on the fiber surfaces. In this work, a dielectric barrier discharge (DBD) source in plane configuration is used to treat flax fabrics in ambient air. It is examined whether it is possible to increase the functionality on both fabric sides, which is achieved by simple changes in the DBD setup. After evaluating the treatment homogeneity of the filamentary plasma, an explanation for the treatment mechanism on the fiber surfaces is developed. It is shown that waxy substances, which naturally occur on natural fibers, play an important role in the wettability of the fabric.