The pitting corrosion of three duplex steels (DSS), SAF2205, SAF2507, Z3CN20.09M thermally aged at 350-500 ℃ for 5500 h was investigated by the means of electrochemical techniques. The pitting potential (E p) of thermally aged Z3CN20.09M specimens decreased with increasing of ageing temperature, however, the E p of SAF2205 and SAF2507 thermally aged specimens decreased first and reached the lowest value at 450 ℃ and then increased slightly. The electrochemical impedance spectroscopy results are coincident with the potentiodynamic polarisation testing results. The variation in the charge transfer resistance of specimens coincided with the E p values. There is little influence of low temperature (below 350 ℃) on resistance to pitting of SAF2205 and SAF2507 DSS. But, the pitting resistance of them degrades drastically ageing at higher temperature above 450 ℃. The deterioration in pitting resistance of thermally aged DSS specimens mainly attributes to the precipitation of Cr-rich α′ phases which affected by temperature markedly. At the range of 350-500 ℃, the temperature the higher, the precipitation of Cr-rich α′ phases the more. The formation of G phases and the healing process during the long thermal ageing at 500 ℃ is the other reasons for the change in pitting resistance.