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This article describes the effect of phosphorus on the microstructure and stress rupture property at 650 degrees C in an Fe-Ni-Cr base superalloy. The results showed that phosphorus markedly improved the intergranular precipitation in the range of 0.0005 to 0.016 wt pet, which facilitated M(23)C(6) and M(3)B(2) precipitation but inhibited the formation of MC carbide. A too high phosphorus addition (0.051 wt pet P) resulted in an excessive precipitation at grain boundaries, while a too low phosphorus content (0.0005 wt pet P) led to many precipitate-free grain boundaries. Phosphorus also enlarged the size of the gamma' particles and lowered its stability, that eta-Ni3Ti preferred to form in the alloy with 0.051 wt pet P. Due to the improvement of the microstructure, appropriate amount of P content significantly prolonged the rupture life of the alloys in the range of 0.0005 to 0.016 wt pet. The peak value was 660 hours at 0.016 wt pet, more than 4 times that of the alloy with 0.0005 wt pet phosphorus, but phosphorus reduced the fracture elongation. The mechanism by which phosphorus influenced the alloy is discussed.