In this study, alkali and isocyanate surface modifications were applied to flax fiber (FF) to improve its adhesion to bio‐based thermoplastic polyurethane (TPU) matrix. In addition to these treatments, isocyanate treated FF was subjected to curing process. TPU/FF composites were prepared at a constant 30 wt% loading of the total by using melt‐blending method. Their mechanical properties, modulus of elasticity, melt‐flow, water uptake and morphological properties were investigated. All of the surface modifications resulted in better mechanical properties with respect to untreated FF. Cured isocyanate treated FF loaded composite exhibited the best results in the case of tensile strength, Young's modulus and storage modulus. Isocyanate treatments caused reduction in melt flow rate due to enhancement in interfacial interactions between phases. It was observed from the SEM micrographs that surface treated fibers dispersed more homogeneously in the TPU matrix. Results confirmed that surface modifications improved the adhesion of FF to TPU matrix. POLYM. COMPOS., 38:2874–2880, 2017. © 2015 Society of Plastics Engineers