Positron annihilation spectroscopy (PAS) techniques, Mossbauer spectroscopy (MS) and transmission electron microscopy (TEM) were applied in the investigation of thermally treated reactor pressure vessel steels (RPV). Results of these methods are discussed and correlated with hardness measurement. PAS and MS measurements have been performed also on the original RPV specimens which are used in frame of the "Extended surveillance specimen program" at the 3(rd) and 4(th) units of NPP Jaslovske Bohunice (Slovakia). PAS angular correlation (1D-ACAR) and lifetime spectra (PLEPS) showed that the degradation of the steel properties caused by neutron irradiation can be detected. General discussion about the contribution of PAS to micro structural study of RPV-steels is included as well.