To control the solidification process during industrial polymer melt processing, it is crucial to understand the effect of adding additive agents, such as a filler, a plasticizer, a nucleating agent, and so on, to a polymer on its crystallization behavior under the rapid cooling condition. The crystallization behavior of polypropylene (PP) / hydrophobic-modified cellulose nanofiber (CNF) composite was investigated using fast scanning calorimetry (FSC). FSC was very useful to reveal the crystal nucleating effect of hydrophobic modified CNFs because it allowed us to investigate the change of crystallization temperature range in the wide range of cooling rate, and the crystallization kinetics in the wide temperature range.