Methanosarcina barkeri (M. barkeri) can cause microbiologically influenced corrosion (MIC) of copper in heat exchangers, where ferric ion (Fe3+) and ferriferous oxide (Fe3O4) also exist due to the corrosion of iron pipes with flowing water. It is worth noting that M. barkeri can reduce Fe3+ to Fe2+ or Fe, which may exhibit an unknown effect on copper corrosion. In this study, the molecular mechanism of copper corrosion by M. barkeri with Fe3+ and Fe3O4 was explored. In the presence of Fe3+, the corrosion was enhanced by 82.5 %, and M. barkeri and Fe3+ exhibited a synergistic effect on copper corrosion. It is attributed to that Fe3+ oxidized the copper and promoted the growth of M. barkeri and biofilm formation, thus promoting corrosion. Moreover, the corrosion by M. barkeri with Fe3O4 increased by only 27.7 %, and M. barkeri and Fe3O4 exhibited an antagonistic effect on the corrosion. Fe3O4 covered the coupon surface, which reduced the contact between the cells and the coupon, and thus inhibited electrochemical corrosion. This study provides new insights for methanogens-induced MIC, and also has far-reaching implication for the mitigation of MIC in heat exchangers.