The effect of grain size, ranged from 37 to 1220 μm, on creep behavior of Hastelloy X was studied at temperatures of 950, 1 000 and 1 050°C. A steady state creep rate, εs, of the alloy decreased with the increase of the grain size, reached a minimum at the grain size of 100200 μm, LM, and increased with the increase of that. From measurements of friction stress (σf) and observations of microstructures, it is concluded that the increase in εs under a given applied stress with the decrease of the grain size is attributable to the increase of an effective stress, σe(=σa-σf, whereey σa is the applied stress). On the other hand, recrystallization induced by a high stress concentration at grain boundary triple points in the coarse-grained alloys seems to accelerate the creep. By comparing the results in the Hastelloy X with those in carbon free 17Cr-14Ni steel, it is suggested that the grain size dependence of the εs of the Hastelloy X having finer grains than LM is more remarkable than that of carbon free 17Cr-14Ni steel because of the decrease of fine precipitates within grains in the former alloy, while the dependence of the alloy having coarser grains than LM is minor than that of carbon free 17Cr-14Ni steel because of preventing the stress concentration at the triple points by the grain boundary precipitates.