A new Ni-Fe based wrought superalloy for 700℃ advanced ultra-supercritical steam turbine rotor application was developed. The effects of B and P on the microstructure and mechanical properties of the alloy were investigated. The results show that the major precipitates in the alloy were γ′, MC, M23C6 and M3B2. B restrains the precipitation and growth of the M23C6, increasing the binding force of grain boundaries and obstacles intergranular crack initiation. P accelerates the precipitation and growth of the M23C6, and suppress crack initiation and propagation in the surface. B and P play an even more significant role in strengthening grain boundaries at 700 ℃ than at room temperature, they improve not only the strength but also the high-temperature ductility of the alloy. But However, high B content impairs the beneficial effect of P element. reducing the tensile strength and rupture life of the alloy.