The goal of this study was to explore the reinforcement capability of micro and nano-size fibers from wheat straw. Microfibers were obtained by alkali treatment and disintegration processes of the wheat straw while nanofibers were obtained after applying further mechanical treatment of this alkali treated wheat straw. The morphology and thermal properties of both fiber types were determined to show their suitability as reinforcements. TEM images showed that the diameters of the wheat straw fibers were decreased from micro to nano-size by the defibrillation process. Thermogravimetric analysis showed the alkali treatment dramatically increased the thermal properties of the wheat straw fibers. The composites were produced using, respectively, the microfibers and nanofibers as reinforcement, with both polyvinyl alcohol (PVA) and cellulose acetate butyrate (CAB) as the matrix. The morphology and thermal properties of the composites were investigated by scanning electron microscopy and thermogravimetric analysis. The mechanical properties of the composites were compared with those of neat polymer matrix and found to be considerably improved.