Cellulose nanocrystals (CNCs) were fabricated from sulfuric hydrolysis of cotton. The CNCs were modified with TEMPO/NaBr/NaClO oxidization and quaternary ammonium salts adsorption. Polystyrene-CNC composite films were prepared with solution casting method. The results of characterization revealed that the thermal properties of CNCs and the compatibility of CNCs with polystyrene were improved, while the morphology and the crystalline structure of the modified CNCs were seldom broken. The contact angle of the modified CNCs was increased from 35.8A degrees of the pure CNCs to 54.6A degrees of CNCs modified with stearyltrimethylammonium chloride, and SEM revealed that the modified CNCs dispersed evenly in the polymeric matrix. In addition, the breaking strength and elongation at break of PS/CNC composites were increased. The study shows that the combination of TEMPO-mediated oxidization and quaternary ammonium's adsorption is an effective method for improving the thermostability and compatibility of CNCs in organic matrix.