Biological conversion of CO2 was examined in a fixed bed reactor inoculated with anaerobic mixed culture to investigate influencing factors, the type of packing material and the composition of the feeding gas mixture. During the operation of the fixed bed reactor by feeding the gas mixture (80% H2 and 20% CO2 based on volume basis), the volumetric CO2 conversion rate was higher in the fixed bed reactor packed with sponge due to its large surface area and high mass transfer from gas to liquid phase compared with PS ball. Carbon dioxide loaded into the fixed bed reactor was not completely converted because some of H2 was used for biomass growth. When a mole ratio of H2 to CO2 in the feeding gas mixture increased from 4 to 5, CO2 was completely converted into CH4. The packing material with large surface area is effective in treating gaseous substrate such as CO2 and H2. H2, electron donor, should be providing more than required according to stoichiometry because some of it is used for biomass growth.