<p>Isotactic polybutene-1 (iPB-1) possesses excellent mechanical performances including good creep resistance at high temperatures, whereas its applications are heavily hindered due to the slow II-I transition kinetics after melt crystallization into form II during melt processing. Though the traditional solid–solid II-I transition mechanism has been widely accepted, with more and more new experimental observations arising, it encounters challenges and is still a controversial topic in polymer physics. After stearic acid-modified calcium carbonate (CaCO₃) nanoparticles (NPs) were melt-blended with iPB-1, no influence on the crystalline structure of as-crystallized form II was observed, but the following entire II-I transition was retarded. It provides a model system for II-I transition study to have a look into its mechanism. The results showed that II-I transition of iPB-1 in iPB-1/CaCO₃ NPs composite can be decomposed into a prior inhibited initial start and a following almost fixed development process of II-I transition. The initial start of II-I transition is triggered by the direct form I formation in the amorphous phase. The more CaCO₃ NPs well-dispersed in the amorphous phase of iPB-1 as obstacles, the more difficult for the confined iPB-1 chain segments to adjust their chain conformations due to the more inhibited chain segmental dynamics. Therefore, the less form I direct crystallized in the amorphous phase, the more retarded II-I transition kinetics of iPB-1 can be observed. The results would shed a light on understanding of the II-I transition mechanism of iPB-1.</p> Graphical abstract <p></p>