The electron transfer mechanisms of yeast Saccharomyces cerevisiae employing two different mediators, methylene blue (MB) and methyl red (MR), are suggested. The effects of the mediators on Microbial fuel cells (MFCs) performances are investigated when yeast and glucose are the biocatalyst and the substrate, respectively. Yeast tends to stand as floating cell rather than attached to supporting electrode. Therefore, to combine direct and mediated electron transfer mechanisms of yeast, two mediators and carbon felt modified with polyethyleneimine (PEI) (CF-PEI) are adopted and their roles are evaluated. As a result, CF-PEI surface is functionalized with amino groups that can attract and entrap more yeast cells. The cyclic voltammetry (CV) curves representing the mechanisms demonstrate that electron transfer rate constant of MB (0.44 s(-1)) is higher than MR (0.37 s(-1)). In addition, the performances of the yeast-MFC adopting MB (429.29 +/- 42.75 mWm(-2) at similar to 1200 mAm(-2)) are better than those of the yeast-MFC adopting MR and the yeast-MFC without mediator. The reason is that MB is effectively adsorbed by yeast and collects more electrons than MR. These benefits of MB are reflected in a more efficient electron transfer chain and minimize the side reactions deactivating the catalyst.