Abstract: The effects of heat treatment (solid solution and aging) on microstructure of base metal and surfacing welding heat affected zone (HAZ) of 2507 DSS (duplex stainless steel) by the means of optical microscopy and scanning electron microscopy. The results show that, after the steel was solid soluted at 1 050-1 200 ℃, the volum fraction of ferrite in base metal increased from 46% to 58% with increasing of the solid solution temperture, and the content of austenite increased. The sustenite grew perpendicular to fusion line in surfacing welding HAZ, and some black precipitated phases were found at the austenitic grain boundaries. σ phase appeared in both base metal and HAZ after aging at 850 ℃, and interlaced needle-shaped γ2 phase could be found in ferrite base in HAZ. Continous and point like carbides could be found at the austenitic grain boundaries in HAZ, and σ phase was not found in base metal and heat affected zone after aging at 620 ℃.