The extensive researches of biodegradable polymer and biocomposites have attracted great interest in various industrial fields with a green concept. The Cellulose nanocrystals (CNCs) are ideal reinforcements for biopolymer composites because of their high mechanical strength, lightness, and stiffness, as well as biodegradability. However, strong intermolecular interaction of cellulose causes aggregation during the preparation of the nanocomposite due to their hydrophilic surface and high surface area, resulting in inefficient reinforcing effect of CNC in the nonpolar polylactide matrix. In this research, the effective surface modifications of CNC are conducted to convert hydrophilic groups into hydrophobic acetyl and alkyl groups by nucleophilic substitution reaction and the degree of substitution is determined by titration. The contact angle analysis by using Owen-Wendt equation indicated that the surface modifications increased remarkably the hydrophobicity of CNCs by decreasing the polar force. The polylactide nanocomposites reinforced by functionalized CNCs were prepared by solution casting method and the effects of surface modified CNC on the thermal stability, morphology, transmittance, and mechanical properties have been investigated. Furthermore, the theoretical prediction in the modulus of polylactide nanocomposites was additionally analyzed to clarify the reinforcing potential of surface modified CNC. The Voight-Reuss and Halpin-Kardos models were used to simulate the modulus of the composites and compared with the experimental values. This study accounts for the effect of surface modified CNC on polylactide matrix, providing a criteria for proper functionalization of CNC as a bio-based nanofiller in polymer nanocomposites. This research is supported by Korea Institute for Advancement of Technology (KIAT) (Project No. N0000993) and National Research Foundation of Korea (NRF) (Project No. 2013-022313)