The influence of aging temperature (200 to 800 °C) on grain boundary segregations in a Ni-20Cr-18W-1Mo superalloy was investigated by scanning electronic microscope (SEM), transmission electron microscopy (TEM), electron microprobe analyzer (EMPA) and mechanical testing machine. Results indicate that the grain boundary segregation critical times of sulfur and phosphorus decrease with increasing aging temperature. Increasing of aging temperature has a conspicuous effect on the concentration distribution between the grain boundary and grain core. Grain boundary concentrations of sulfur and phosphorus increase with increment of testing temperature until a peak value obtained at 650 and 400 °C, respectively, which is the essential reason to the declined mechanical properties from 200 to 600 °C.