Ni–MgO nanoparticles (NPs) encapsulated by porous silica shell for CO₂ methanation were synthesized. The effects of Ni/Mg ratios in core–shell structured catalysts on CO₂ conversion and CH₄ selectivity were examined. Results showed that the NiO–MgO@SiO₂ catalyst with a Ni/Mg ratio of 4/1 exhibited the highest catalytic activity (87% conversion for CO₂ and 99% selectivity for CH₄) and catalytic stability for the whole 100 h time on stream at low temperature (300 °C). It was found that the high catalytic activity and stability of this catalyst could be attributed to the isolated highly dispersed Ni NPs, which were obtained through the reduction of NiO–MgO solid solution protected by the silica shell.