The crystallization process of a folded-chain crystal (FCC), the most common crystalline structure of polymer chains, was clearly visualized at the molecular level in situ and in real time for the first time. The sample was an isotactic poly(methyl methacrylate) monolayer deposited on mica in an amorphous state, the crystallization of which occurred under high humidity and was followed by atomic force microscopy. Detailed crystallization behaviors, especially the stepwise growth of the FCC with blocks shorter than the chain, cooperative chain slipping in the FCC, the formation of an anisotropic nucleus and its growth, and the formation of a small nucleus with a short lifetime, were clearly visualized at the molecular level. The stepwise growth of the FCC differed from that expected by the classical Lauritzen-Hoffman theory and was consistent with recent reports that indicated the formation of some ordered state in the amorphous phase. We believe this molecular information will improve our understanding of the molecular mechanism of polymer crystallization.