This paper explores the size effect of zirconia particles on the catalysis of Ni metal for CO2 re-forming of methane. It is revealed for the first time that when the sizes of zirconia particles become samller than 25 nm, the oxide forms nanocomposite catalysts with size-comparable Ni-metal nanocrystals (10-15 nm). The nanocomposite catalysts show extremely stable catalysis, which is in strong contrast with the deactivating Ni catalyst supported on bigger zirconia particles (>25 nm). This raises the possibility of tailoring the catalytic behavior of oxide-supported metal catalysts by reducing the particle size of oxide to make high-performance nanocomposite catalysts.