Cellulose nanocrystals (CNCs) were separated from soy hulls by acid sulfuric hydrolysis. TEM, AFM, WAXS, elemental analysis and TGA techniques were used to characterize CNCs. The result shows that the CNCs have a highly crystallinity, specific surface area and aspect ratio. The aspect ratio (around 100) is the largest ever reported in the literature for a plant cellulose source. These CNCs were used as a reinforcing for prepare nanocomposite by casting/evaporation. The mechanical properties were studied in both the linear and non-linear ranges. The result indicated that the reinforcing of CNCs made from soy hulls was higher than CNCs extracted from other sources. These CNCs is not only the highest aspect ratio but also the stiffness of CNCs has percolating nanoparticle network formed within the polymer matrix. Furthermore, the precipitation of CNCs during the evaporation step is a crucial role on the mechanical properties.