Nickel particles in a catalyst for steam methane reforming showed more sintering at lower temperature than at higher temperature with H-2 addition. A Ni(6.8 wt.-%)/alpha-Al2O3 catalyst showed a continuous and irreversible deactivation over 430 h due to sintering of Ni in the presence of 2% H-2 addition at 862 K. At 1017 K, the catalyst also showed an activity decrease with 2% H, addition, but the activity was stabilized at a certain level over 130 h and it recovered after the H, addition was stopped. The nickel sintering corresponded to the activity change in the experiment at 1017 K as well as that at 862 K. The catalyst in the form of 5 mm sphere was short-packed in 10 mm length, and the standard conditions were 9.7 bar, a steam/carbon (S/C) = 3, and W/F = 26 s. The sintering was confirmed by SEM observation. Sintering reference experiments in H-2/N-2 and in H2O + H-2/N-2 under conditions equivalent to those of the above activity measurements showed that carbon-bearing components played an important role in preventing the sintering. Sintering did not occur in H-2/N-2 atmosphere and much more sintering occurred in H2O + H-2/N-2 atmosphere than under the activity measurement conditions. In a long-packing experiment conducted in analogy to an actual plant operation with an axial temperature gradient of 883-980 K, the same phenomena as those in the short-packing experiment were confirmed by SEM observation. However, the deactivation did not appear because in the tube the fraction of the deactivation section where the temperature is low and PHI is high is small. This seems to be the reason why no deactivation by H, has ever been reported in actual plant operations.