Abstract: Some micro-cracks were found in weld metal of P92 steel welded joint in ultra supercritical unit after 8 000 h service, these micro-cracks were observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Morphology, distribution and property of these micro-cracks were analyzed. Results show that both intergranular and transgranular cracks in P92 steel welded joint were presented, and the micro-cracks showed wormy shape with discontinuous distribution. A large number of precipitates were distributed in the front area near the crack tips. The formation of micro-cracks was related with stress and structure of micro-zone, and the micro-cracks derived from low temperature region during welding procedure and formed from interconnection of micro brittle cracks under the influence of stress field.