Under confinement, unique features of crystallization arise, among which the slowing down and accelerating crystallization on cooling are both reported. The controversy can be resolved by performing isothermal crystallization experiments, taking into account temperature dependence of crystal nucleation and growth. The isothermal experiments of thin films were performed for the first time in this work in the wide range of thicknesses and wide range of temperatures, where it crystallizes. Fast scanning calorimetry (FSC, Flash DSC 1) was used to reach fast cooling cooling and heating, avoiding crystallization and unwanted reorganization. Thin films of poly(ethylene terephthalate) (PET) were studied from 23 nm thickness with free surface and covered by a PS layer. Two-step crystallization was observed by FSC in films thinner than 60 nm at temperatures of crystallization below 10 140 degrees C. Evidence of strong free-surface-induced precrystallization was confirmed by atomic force microscopy (AFM). It was observed that highly mobile surface layer formed dendritic crystals first and then bulk layer formed spherulites. When we used thicker films or films crystallized at low undercoolings, no two-step crystallization was observed neither in FSC nor in AFM, presumably due to domination of bulk crystallization. Heterogeneous nucleation at the solid interface with crystalline poly(ether ether ketone) (PEEK) covering layer enhances crystallization at high temperatures. These results not only help to rationalize contradictory reports in the literature but also provide a strong methodology for study of crystallization and nanotechnology.