Precipitation of Cu-rich clusters/nano-phase is one of the main reasons concerning irradiation-induced embrittlement of reactor pressure vessel (RPV) steel.In order to investigate the precipitation of Cu-rich clusters/nano-phase and the evolution of their crystal structure, content of Cu in RPV model steels is raised to 0.6wt.%.The RPV model steel was quenched and tempered at 660℃,then thermal aged at 400℃for 200,1000,2000, 4000and 13000h.The number density and equivalent diameter was observed by atom probe tomography (APT) and the crystal structure was investigated by high resolution transmission electron microscopy (HRTEM).Results show that with extending of the aging time, the size of Cu-rich nano-clusters increases gradually, together with an increase of Cu content and decrease of the Fe content; the number density reaches the highest when aging for 1000h and then declines.The growth and coarsening of Cu-rich clusters/nano-phase consist with LSW theory well.The initial structure of Cu-rich nano-phase is of b.c.c structure and then transform into twin 9R with the further segregation of Cu atom.Because of the effect of interfacial energy of Cu-rich phase/matrix and twin crystal and elastic strain energy in Cu-rich phase, 9R, f.c.t, and f.c.c structure were observed in one single Cu-rich phase in the meantime.The complex process from metastable to stable structure demonstrate that besides the common precipitation sequence of b.c.e→twin 9R→non-twin 3R (f.c.t.)→non-twin f.c.c., there may be other two new ones of Cu-rich nano-phases, which are b.c.e→non-twin 9R→twin f.c.cand b.c.c→twin 9R→twin 3R (f.c.t.) →twin f.c.c.However, it seems that no correlation presents between size and crystal structure of Cu-rich phase and Cu content.