A series of fiber electrodes with fiber diameters ranging from about 10 to 0.1 mu m were tested as anodes in microbial fuel cells to study the effect of fiber diameter on the behavior of biofilm and anodic performance of fiber electrodes. A simple method of biofilm fixation and dehydration was developed for biofilm morphology characterization. Results showed that the current density of fiber anodes increased until the fiber diameter approached 1 mu m which was about the length of the dominant microorganisms in biofilm. The highest current density was 3.08 mAcm(-2), which was obtained from fiber anode with high porosity of over 99% and fiber diameter of 0.87 mu m. It was believed that the high current density was attributed to the high porosity, as well as proper fiber diameter which ensured formation of thick and continuous solid biofilms. (C) 2011 Elsevier Ltd. All rights reserved.