The electrochemical limitations on the performance of microbial fuel cells (MFCs) are mainly due to the internal resistance. The total resistance in the MFC was expressed as the apparent internal resistance (R(i)) which was partitioned into ohmic resistance (R(omega)) and non-ohmic resistance (R(n)), referring to the equivalent circuit of the MFC. In the one-chamber MFC, R(i) and R(omega) were measured using the steady discharging method and the current interrupt method, and they were 289 omega and 99 omega, respectively. The maximal power density was 241 mW/m2 when the external resistance equaled to the apparent internal resistance. The stabilization time of 60 s was enough to remove the influence of the capacitors in the steady discharging method. When the MFC was in the activation overpotential area, the ohmic overpotential area and the concentration overpotential area respectively, R(n) accounted for 93%, 66% and 75% in R(i). The ratio of R(n) to R(i) was the lowest when power output of the one-chamber MFC reached its highest value. Decreasing R(n) and R(omega) is the key to one-chamber MFC's output increasing.