Macroscopic and microscopic structural changes of Ni/YSZ cermets in CH4, C2H6, and C3H8 containing atmosphere at 1073 K were evaluated by dilatometry and SEM observation. Carbon deposition was observed in low-humidified hydrocarbon atmospheres, accompanying significant expansion of the cermet. SEM observation of the cross section revealed that carbon deposition took place preferentially near the surface and Ni particles seemed to be surrounded by the deposited carbon. The expansion due to carbon deposition did not depend on the kind of hydrocarbon but on the amount of supplied carbon. On the other hand, in high-humidified hydrocarbon atmospheres, e.g. S/C ≥ 1, no carbon deposition and expansion were observed.