The main drawbacks of flax fibers have been attributed to poor compatibility with polymer matrices as well as relatively high water absorption. The aforementioned properties are mainly due to the presence of hydrophilic hydroxyl functional groups on the backbone of the flax fibers. This study aims to convert primary alcoholic (OH) groups on the surface of flax fiber to carboxyl groups by using TEMPO oxidation in order to facilitate the silane treatment process. Subsequently, carboxyl groups can more easily interact with silane coupling agents. The surface functionality of as-received and treated fibers was characterized using Fourier transform infrared and X-ray photoelectron spectroscopy. Dynamic contact angle tensiometer was used to compare wettability of the oxidized and non-oxidized fibers after the silane treatment. The interaction between flax fiber and polymer was characterized using scanning electron microscopy (SEM). The results indicated that the TEMPO oxidation significantly improved the bonding efficiency of the silane coupling agents on the fiber surface. Thus, the compatibility between the flax fibers and the epoxy resin was improved. In addition, the water absorption of the modified fibers was remarkably reduced, while the contact angle of the flax fibers was increased.