The micromechanical properties (microindentation hardness, H, elastic modulus, E) of poly(ethylene terephthalate) (PET), isothermally crystallized at various temperatures (Ta) from the glassy state are determined to establish correlations with thermal properties and nanostructure. Analysis of melting temperature and crystal thickness derived from the interface distribution function analysis of SAXS data reveals that for Ta190 °C a population of lamellar stacks with a unimodal thickness distribution emerges. The H and E-values exhibit a tendency to increase with the degree of crystallinity. The results support a correlation E/H∼20 in accordance with other previously reported data. The changes of microhardness with annealing temperature are discussed in terms of the crystallinity and crystalline lamellar thickness …