The microstructure and properties of UNS S31803 duplex stainless steel had been investigated following isothermally aging in a temperature range of 450–1000°C for 10min, which simulates a fairly long time relative to a weld thermal cycle. The results showed that specimens aged between 600 and 950°C are subjected to precipitation of secondary phases, such as Cr 2 N, σ and χ. And the most significant reduction in pitting corrosion resistance and impact energy occurs at 850°C. Moreover, at aging temperatures below 600°C, austenite is the preferred site for pitting initiation, which changes to precipitates modified ferrite phase at higher temperatures above 600°C.