This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe(3)O(4), 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10-4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe(3)O(4) and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 +/- 0.5 (control), 12.5 +/- 0.8 (nFe(3)O(4)-added), and 15.7 +/- 1.1 mu S/cm (CNT-supplemented). Dissolved iron concentration, released from nFe(3)O(4), seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe(3)O(4) and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe(3)O(4) and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.