Blends of Polyamide-6/Poly(phenylene oxide) (PA-6/PPO) were prepared by in situ polymerization, in which the reactive compatibilizer SP was added. Based on two kinds of kinetic equation of nonisothermal crystallization proposed by Ozawa and Liu, the influences of PPO, the cooling rate, and the compatibilizer on crystallization process of PA-6 were investigated. At a given cooling rate, the presence of PPO reduces the overall crystallization rate of PA-6; for a fixed PPO level, the time of crystallization completed becomes shorter when the cooling rate is higher; the addition of SP impedes the development of crystal growth. Scanning electronic microscope (SEM) results fortified the above conclusion. According to the analysis result of experiment data, it shows that the Ozawa equation does not adequately describe the nonisothermal crystallization behavior of PA-6/PPO blends, whereas the Liu approach can be well applied in this studied system.