The Cu-containing austenitic heat-resistant steel 18Cr9Ni3CuNbN, which is being used as superheater and reheater tube material for modern ultra-super-critical (USC) power plants all over the world, has been investigated at 650℃ long time aging till 10 000 h. SEM, TEM and 3DAP (three dimensional atom probe) have been used to follow microstructural changes with mechanical property variations. Experimental results show that Cu-rich phase and MX precipitate in the grains as well as M₂₃C₆ precipitates at grain boundaries are the main precipitation strengthening phases in this steel. Among them Cu-rich phase is the most important strengthening phase. Homogeneous distribution of very fine nano-size Cu-rich phase has been formed at very early stage of 650℃ aging (less than 1 h). Cu atoms gradually concentrate to Cu-rich particles and the other elements (such as Fe, Cr, Ni etc) diffuse away from Cu-rich particles to γ-matrix with the increasing of aging time at 650℃. The growth rate of Cu-rich phase at 650℃ long time aging is very slow and the average diameters of Cu-rich phase have been determined by TEM method. Cu-rich phase keeps in about 30~nm till 650℃ aging for 10 000 h. It shows that nano-size Cu-rich phase precipitation strengthening can be kept for long time aging at 650℃ because of its excellent stability at high temperatures. According to structure stability study and mechanical properties determination results the Cu-rich phase precipitation sequence and its strengthening mechanism model have been suggested and discussed.