The microindentation hardness (H) of PET samples crystallized isothermally at various temperatures from the melt under high pressure (4kbar) was determined in order to establish correlations with the thermal properties and microstructure. The results reveal that these materials show density, enthalpy of fusion, and melting temperature values which are close to those corresponding to chain extended crystals. The high crystallinities obtained (alpha = 0.70-0.95) for high pressure crystallized PET are shown to give rise to unprecedentedly high microhardness values, ranging between 300 and 400 MPa. The data are analysed on the basis of a two-phase model which allows an estimation of the influence of crystal thickness upon H. Finally, the temperature dependence of high pressure crystallized PET is also examined.