Abstract: A series of NiO@SiO₂ core-shell catalysts were prepared using modified Stöber-method. Their catalytic performances in methanation of syngas were investigated in slurry reactor at 320 ℃. The catalysts before and after reaction were characterized by XRD, TEM, XPS, N₂-physisorption, etc. It was found that the NiO@SiO₂ core-shell samples had well-shape morphologies and relatively uniform scale. The catalyst test revealed that the methanation activity of these catalysts decreased dramatically with increase of core particle size. The three catalysts with distinct size of core and shell showed remarkably rapid deactivation in the initial period of 20 h and then deactivated slowly during the following reaction, while their CH₄ selectivity maintained at about 80%. Void-shell was formed during the reaction probably because easy-migrated Ni (CO)_x species were generated. Apparently, it was concluded that increase of core particle size, decrease of BET surface area and pore volume, and abatement of mesopores within 3-5 nm in the shell were responsible for the deactivation of these core-shell catalysts based on the characterization of catalysts.