Abstract: There were three ways to reduce metal oxides by metal reducing microorganisms:direct contact, electron shuttling and chelation of insoluble metal oxides. A microbial fuel cell by Geobacter metallireducens was used to investigate the influence of the three paths on the rate of dissimilatory iron reduction. The results indicate that in the process of dissimilatory metal reduction, NTA and AQDS can accelerate the speed of ferric oxides reduction and magnetite generation in the initial stage, but magnetite production prevents further iron reduction. Direct contact is a very important way, and the biomembrane formed by adsorption for long time is a critical factor for reduction. Electron shuttling plays a less important role after most of mineral oxides surfaces have been enclosed by microorganisms.