The initiation and growth of micro-defects such as micro cracks and voids usually causes the failure of long term operated structural components at high temperature. In this study, the creep characteristics and void nucleation and growth characteristics of P92 steel which is used as main steam pipe material in power plant were investigated at several temperatures and loading conditions. The area fraction of void increased with increase of test temperature, stress, and load holding time. In case of internal defect presence, micro-voids initiated in the early stage of loading period and resulted in the increased load line displacement and crack growth rate. The microvoids were found to form along the prior austenite grain boundaries and at the martensite packet boundaries.