The information regarding phase changes and structure transformation, particle sizes as well as active species of the catalyst are briefly discussed towards alumina supported Ru/MnO catalysts according to their various parameters of calcination temperatures and Mn loading. The Ru/Mn-75/Al₂O₃ catalysts calcined at 400°C, 700°C, 900°C, 1000°C and 1100°C, Ru/Mn-65/Al₂O₃ and Ru/Mn-85/Al₂O₃ catalyst calcined at 1000°C were synthesized by the wetness impregnation method. All the prepared catalysts exhibited crystallite size in the range of 95 nm to 114 nm. It was found that the catalyst with Ru/Mn-75/Al₂O₃ calcined at 1000°C showed the highest 60.21% CO₂ conversion with 57.84% formation of CH₄ at the reaction temperature 200°C. The expected active species that assist the CO₂ methanation activity over this catalyst was Mn₃O₄.