The properties of polymer ultrathin films are a subject of intense study from both practical and academic viewpoints. Previously, we found that upon compression, an isotactic poly(methyl methacrylate) (it-PMMA) Langmuir monolayer crystallized to form a two-dimensional (2D) folded-chain crystal, and the molecular image of the crystal with chain folding and tie chains was dearly visualized by atomic force microscopy (AFM). In the present study, the melting behaviors of the it-PMMA 2D crystals were successfully observed in situ by high-temperature AFM at the molecular lever for the first time. The chain chain distances (similar to 1.2 nm) of the crystals were dearly resolved even at temperatures dose to the melting temperatures (T-m) of the 2D crystals. We found that the T-m of the 2D crystals was at most 90 degrees C lower than the bulk crystals. The T-m depression strongly depended on the molecular weight, while the molecular weight dependence of the bulk T-m was negligible in the molecular weight regime studied. The T-m depression also depended on the substrates, a slightly larger depression being observed on a sapphire substrate compared to that on a mica. The large T-m depressions of the 2D crystals could not be explained by a simple Thomson-Gibbs argument, theoretical developments are necessary to understand the melting of the 2D crystals.