Abstract High-resolution transmission electron microscopy was used to investigate the G-phase precipitation behavior in duplex stainless steels long-term thermally aged at 300–450°C. Rising temperature can accelerate thermal aging kinetics and shorten the incubation period of G-phase. G-phase adopts a cube-on-cube orientation relationship with the ferrite matrix. Transformation from ferritic cluster to G-phase can be accomplished through smaller atomic position readjustments. After aging at 400°C for 20,000h, G-phase grows up to 4–5nm and the interface between G-phase and the ferrite matrix still maintains semi-coherent. Disarrangement of crystal planes causes misfit dislocations and strain fields around G-phase.