A single-chamber, membrane-less and floating-cathode microbial fuel cells (MFC) was successfully started up using surplus sludge as fuel without adding carbohydrate after 20 days. The electricity generation of the MFC was investigated. The results showed that the obtained maximum voltage was 440.7 mV when external resistance was 1,000 Ω. There were four phases (rapid fall phase, stationary phase, fall phase and stationary phase of low voltage) in the periodic time of MFC, and the output voltage range of 150–300 mV lasted for 107 hours during stationary phase. In addition, the obtained maximum power density was 220.7 mW m−2 and internal resistance was 368.13 Ω in the MFC. Consequently, surplus sludge could be used to generate electricity in MFC, which could provide a novel process for sludge recycling.