Hemp fibers were modified with 3-isopropenyl-dimethylbenzyl isocyanate (TMI), in presence of dibutyltin dilaurate (DBT) as a catalyst. Then reinforced hemp-unsaturated polyester (UPE) composites were prepared with modified fibers by means of hand lay-up compression molding. The fiber treatments significantly increased the tensile strength (TS), flexural strength (FS), and water resistance of the resulting composites, while their flexural modulus (FM) was not influenced. The tensile-fractured surfaces of the composites were observed by SEM and the images revealed that fiber treatments significantly improved the interfacial adhesion between hemp fibers and UPE resins. FT-IR spectra and X-ray photoelectron spectroscopy (XPS) analysis indicated that the treated-fibers were partly covalently bound to TMI.