Specimens of SAF2507 super-duplex stainless steel were heated at 920°C with different aging time. The phase transformation and development of microstructure in the materials were investigated by color optical microscopy, scan electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The hardness was tested by HB-3000B hardness tester. The facts that sigma phase precipitated right after 2 min. At 920°C, the grain boundaries to be the preferential precipitation sites and the sites changed from ferritic-austenitic phase interface into the inside of ferritic phase was found. Increase in aging time is proportional to the increase in sigma phase precipitation mass was most frequently observed in specimens. For phase precipitation, the values of hardness of the super-duplex stainless steel was enhanced evidently.