The changes in the morphology of ’ precipitates and the dislocation substructures at the vicinity of grain boundaries with creep deformation were investigated on a polycrystalline nickel-based superalloy, IN-100. The experiments were done at 1273K in the stress range of 70-180MPa with one set of samples deformed until rupture and another interrupted after reaching the minimum creep rate. The cuboidal ’ precipitates were regularly arrayed at the vicinity of grain boundaries and eutectic ’ precipitates and carbides were observed at the grain boundaries on as-heat treated IN-100. However, on both the creep interrupted and ruptured samples at all stress conditions, plate - like shaped ’ precipitates covered the grain boundaries. The width of the plate - like shaped ’ precipitates increased with decreasing the stress and is wider in the creep ruptured than in the creep ruptured samples. There were few dislocations in the grains and in the plate - like shaped ’ precipitates, but a large number of the dislocations were observed around the interface of the plate - like shaped ’ precipitates. TEM observations on the specimen creep ruptured at 180MPa indicated that strong constant was observed at the vicinity of grain boundaries, but weak contrast in the grain interior. These results suggested that drastic plastic deformation occurred only at the vicinity of grain boundaries. Consequently, the plate - like shaped ’ precipitates at the grain boundaries inhibited the dislocations movement and acted as the creep strengthener.