Nano-sized cellulose fibrils called cellulose nanofibers (CNFs) have been recently attracting a great deal of attention because of their superior characteristics in regard to mechanical strength, light weight and bio-renewable resource, and they are often compound with polymers to improve the mechanical property. When CNFs are compounded with hydrophobic polymers, such as polypropylene (PP), the surface modification of CNF with the hydrophobic functional groups is needed to ensure the good dispersity of CNF in the polymer. The chemical modification of CNF could improve not only the dispersity but also thermal or rheological properties of the polymer. In this study, PP / hydrophobic modified CNF (HM-CNF) composites were prepared and, with the fast scanning chip-calorimetry, the crystallization behaviors of the composites were measured under the practical processing conditions, such as the fast cooling rates conducted by injection molding machines. The effect of the chemical modification on the kinetics of PP crystallization was investigated by changing the length of the alkyl chain of modifier as C2, C8 and C14. The overall crystallization rate of PP/HM-CNF composites was increased at temperature above 40ºC by the presence of HM-CNF. This was because the HM-CNF acted as a nucleating agent and promoted the heterogeneous nucleation of PP crystal. The effect was strongly dependent on the kind of hydrophobic group, the degree of modification and the temperature range. HM-CNFs’ dispersity in PP increased with the length alkyl chain of the modifier, on the other hand, the temperature at which the crystal nucleating effect was clearly observed was lowered. Namely, the crystal nucleating effect became weaker as the hydrophobicity and dispersity of CNFs became higher. The crystallization behaviors at the fast cooling rates were also influenced by hydrophobic modification of CNFs. At the cooling rates from 1 K s-1 to 1000 K s-1, the crystallization temperature of the PP / C2-CNF composite rose more than 10ºC compared with the PP / C14-CNF composite.