Abstract: Isothermal crystallization kinetics of polypropylene (PP) confined in a polypropylene/polystyrene (PP/PS (20/80)) blend (m(PP)/m(PS)=20/80) was studied by fast scanning calorimetry (FSC) at a scanning rate up to 20 000 K/s over a wide temperature region from glass transition temperature to melting temperature. Due to the incompatibility, PP component was dispersed as micron droplets with an average size of 1 μm in the PS matrix. Confined crystallization of PP droplets was observed by conventional differential scanning calorimeter (DSC) with multiple crystallization peaks during cooling. After the addition of nano-SiO₂ particles, no obvious change of the confined crystallization behavior was observed by DSC. With the new technique, FSC, by studying the total crystallization rate of PP at several annealing temperatures, the confined crystallization behavior of PP component in immiscible PP/PS blends was investigated, and one order of magnitude decrease of the crystallization half time of PP was observed, mainly due to the loss of heterogeneity in each PP droplet after dispersing in PS. By adding 2% mass fraction nano-SiO₂ particles in the blend, on one hand, more heterogeneous nucleation sites were introduced for PP component and the crystallization rate of PP was increased at higher temperatures where heterogeneous nucleation was dominating. On the other hand, some nano-SiO₂ particles were located at the interface between the continuous and dispersed phase, lowered the interfacial tension and increased the mobility of PP chains, thus increased the crystallization rate. From the isothermal crystallization kinetics of PP in PP/PS blends with and without nano-SiO₂, it was observed that the addition of nano-SiO₂ particles could partially suppress the confinement effect and increase the crystallization rate and crystallinity of PP in immiscible PP/PS blends, thereby improve the mechanical property of the blends.