In order to improve the optical and mechanical performances of waterborne polyurethane(WPU), nanocrystalline cellulose(NCC)/WPU composites were synthesized in this study. NCC(prepared by acid hydrolysis of cotton fiber) was modified by(3-aminopropyl)triethoxysilane(APTES) to enhance its compatibility with WPU, and the surface-modified NCC was characterized by grafting ratio, crystallinity and contact angle(CA). NCC/WPU composites were examined by scanning electron microscopy(SEM), X-ray powder diffraction(XRD) and thermogravimetric analysis(TG). The anti-yellowing property, specular gloss, pencil hardness, and abrasion resistance of NCC/WPU composites were investigated by the methods of Chinese National Standards GB/T 23999-2009, GB/T 9754-2007, GB/T 6739-2006 and GB/T 1768-2006, respectively. The results showed that the grafting ratio of NCC modified by 6% APTES was 36.01% and the crystallinity of modified NCC was decreased with the enhancement of APTES. CA of the modified NCC was decreased by 28.8% and the nanoparticles were homogeneously dispersed in the WPU matrix. The XRD patterns of the NCC/WPU composites were relatively steady, while the thermal stability of the composites was enhanced by 6.7% with 1.0 wt% modified NCC. Modified NCC affected the specular gloss of NCC/WPU composites more obviously than the anti-yellowing property. The pencil hardness of NCC/WPU composites was increased from 2H to 4H by addition of NCC and the abrasion resistance of the composites was enhanced significantly. In general, NCC/WPU composites showed significant improvements in the optical and mechanical performances. In order to improve the optical and mechanical performances of waterborne polyurethane (WPU), nanocrystalline cellulose (NCC) / WPU composites were synthesized in this study. NCC (prepared by acid hydrolysis of cotton fiber) was modified by (3-aminopropyl) triethoxysilane APTES) to enhance its compatibility with WPU, and the surface-modified NCC was characterized by grafting ratio, crystallinity and contact angle (CA). NCC / WPU composites were examined by scanning electron microscopy (SEM) ) and thermogravimetric analysis (TG). The anti-yellowing property, specular gloss, pencil hardness, and abrasion resistance of NCC / WPU composites were investigated by the methods of Chinese national standards GB / T 23999-2009, GB / T 9754-2007 , GB / T 6739-2006 and GB / T 1768-2006, respectively. The results showed that the grafting ratio of NCC modified by 6% APTES was 36.01% and the crystallinity of modified NCC was decreased with the enhancement of APTES. CA of the modified NCC was dec reased by 28.8% and the nanoparticles were homogeneously dispersed in the WPU matrix. The XRD patterns of the NCC / WPU composites were relatively steady, while the thermal stability of the composites was enhanced by 6.7% with 1.0% by weight modified NCC. Modified NCC affected the specular gloss of NCC / WPU composites more obviously than the anti-yellowing property. The pencil hardness of NCC / WPU composites was increased from 2H to 4H by addition of NCC and the abrasion resistance of the composites was significantly significantly. In general, NCC / WPU composites showed significant improvements in the optical and mechanical performances.