Polypropylene (PP)/ CaCO3 composites were prepared by internal mixer, whose torque reflected the process of material-loading, melting and mixing balance as well as filling effects in viscosity. DSC, polarized microscopy and mechanical tests showed that nano-particle induced heterogeneous nucleation incrementing crystallization, which resulted in weak phase interface and more smaller crystals capable of absorbing dynamic energy being illustrated by higher impact strength. In contrast, too high content of CaCO3 easily aggregated relaying crystallization corresponding to lower strength. In comparison, crystallization of PP/CaCO3 in one pressure-improving equipment was successfully monitored by ultrasonic wave over wide temperature and pressure ranges. The ultrasonic velocity dropped for more CaCO3 in composites when crystallization transition moved to lower temperature. In addition, higher pressure improved this transition temperature quickening crystallization and solidification.