In industrial processes, various additives are used for several purposes, such as modification of mechanical, thermal and rheological properties, oxidation prevention, and improvement of moldability. When the additives are used to semi-crystalline polymers, the crystallization behavior of the polymers is subject to change. However, the effects of additives on the crystallization of semi-crystalline polymers have not been well understood, especially under the rapid cooling conditions that industrial processing machines provide. In this study, we focused on polyhedral oligomeric silsesquioxane (POSS) as a nano-scale size additive, which has an organic functional group at each apex of the silica cubic skeleton. It has been reported that the addition of POSS to polymers improves thermal stability and mechanical properties. We investigated the effect of the additive on the crystallization behavior of isotactic polypropylene (iPP) in the rapid cooling conditions using fast scanning calorimetry (FSC). We measured the overall crystallization rates at the isothermal conditions in the temperatures range 0 - 125 ºC and found exciting effects of POSS: the POSS acts as a crystallization retarder below 80ºC, but it acts as an accelerator above 90ºC.