How to acclimate a highly electroactive biofilm in short time is the bottleneck to improve the power output of microbial fuel cells (MFCs). Here we demonstrated that a simple method, the gravity settling (GS) of planktonic bacteria, is cost effective to improve MFC performance instead of physical and chemical treatment of anodes. The startup time is 12% shorter, and the maximum current density increases by 29% to 8.41 +/- 0.13 A m(-2) than that of the control. Cyclic voltammetries at different growth stages show that GS has a remarkable improvement (66%) on limiting current at the lag stage than at exponential (32%) and mature stages (24%), which was due to the 73% decrease in charge transfer resistance. Biofilm analysis further reveals that the GS promotes biofilm electroactivity per protein in addition to the accumulation of more biomass by gravitational settling, especially at the very beginning of electroactive biofilm formation. Our findings provide new knowledge on MFC startup, which is also important to enhance power densities of large scale MFCs in the future. (C) 2017 Elsevier Ltd. All rights reserved.