In this study, several treatments including alkali, permanganate, peroxide and silanization were applied to flax fiber (FF) to improve its adhesion to bio-based thermoplastic polyurethane (TPU) matrix. TPU/FF composites were prepared at a constant ratio of 30 wt% by using extrusion process. Their mechanical properties, modulus of elasticity, water uptake behaviors, melt-flow properties and morphologies of composites were investigated. All treatments resulted in better mechanical properties with respect to untreated FF. Silane treated FF containing composite exhibited the highest tensile strength and lowest water absorption. Results confirmed that surface modifications improve the adhesion between FF and TPU matrix.