This study presents the effects of nanoscale zero valent iron (nZVI) concentration on the biomethanation of gaseous CO2. During anaerobic batch experiment with 9 times injection of CO2, the CO2 concentration in the headspace rapidly decreased by dissolution. Then, when nZVI was added at 6.25 and 12.5 g/L, the dissolved CO2 was biochemically transformed into CH4 at a maximum production rate of 2.38 and 3.93 mu mol/hr, respectively. Biomethanation at these two nZVI concentrations continued until the end of experiment. In spite of more H-2 evolution by nZVI at 25 g/L, biomethanation did not occur, due to the significant inhibition of methanogenesis by nZVI. As the nZVI concentration increased, relative abundance of the hydrogenotrophic methanogens, especially Methanobacteriales, increased. However, at 25 g/L of nZVI concentration, acetic acid was accumulated and the relative abundance of Clostridium became predominant, indicating that homoacetogenesis was superior over methanogenesis.