Abstract: Microstructure and mechanical properties changes of NF709R steel after long time aging at 700 ℃ were studied by microstructure analysis and mechanical properties test. The results show that the main precipitation phases in NF709R steel were M23C6, MX and Z at 700 ℃ aging. During 4-3 400 h, the high temperature yield strength of the steel raised gradually because of the fine MX phase in grains, Z phase and M23C6 in grain boundaries; and during 3 400-6 000 h aging, the high temperature yield strength decreased because of the loosening M23C6 which weaken grain boundaries strength; and during 6 000-10 000 h aging, the high temperature yield strength of the steel decreased slowly and all precipitation changed little. The M23C6 phases which grew up on the grain boundaries, gathered into chain and widened the grain boundaries resulted in the room ductility and toughness of NF709R steel as aging decreased.