Cellulose nanofiber (CNF), derived from renewable resources, has a great potential of mechanically-reinforcing thermoplastic products, especially polymer foams. The presence of internal hydrogen bonding provides the high modulus, but it reduces the dispersion of CNF in polymer because of its hydrophilicity. To assure the expected reinforcement performance of CNF, the high dispersion of CNF in polymer is substantially needed. One of the methods of controlling the dispersion of CNF in the thermoplastic polymers is to modify CNF chemically and increase the hydrophobicity of CNF. The purpose of this study is to investigate the effects of the hydrophobic modification and the kneading conditions on crystallization behavior of polypropylene (PP) / CNFs composites. The acetylation of CNF was conducted with acetic anhydride at different degree of substitution (DS). The acetylated cellulose micro-fibrils were compounded with PP powder and nano-fibrillated with shear strain in a twin-screw kneader at 180ºC. The compounding was performed at different periods of kneading time to see the effect of kneading condition on dispersion and crystallization. The isothermal crystallization behaviors of the prepared PP / Ac-CNFs composites were measured in the range between 0ºC and 110ºC with the fast scanning chip-calorimetry (FSC). Moreover, the morphology of PP crystals was observed with a polarized optical microscope. Blending Ac-CNFs with PP increased their overall crystallization rate of PP at the temperatures higher than 40 ℃, where the heterogeneous nucleation dominates crystallization kinetics. The acceleration factor, which was defined as the ratio of half crystallization times of PP to PP with the crystal nucleating agent, was dramatically changed by the DS of Ac-CNFs. Morphological analysis with polarized optical microscopic images indicated that the dispersion and fibrillation of Ac-CNFs were significantly affected by the periods of kneading time. However, the acceleration