Batch anaerobic digestion was employed to investigate the performance of the floatable oil (FO) skimmed from food waste (FW) and the effect of different FO concentrations (5, 10, 20, 30, 40 and 50 g/L) on bio-methane production and system stability. FO and FO + FW were mono-digested and co-digested. The results showed that FO and FO + FW could be well anaerobically converted to biomethane in appropriate loads. For the mono-digestions of FO, the biomethane yield, TS and VS reduction achieved 607.7-846.9 mL/g, 69.7-89% and 84.5-92.8%, respectively, when FO concentration was 5-40 g/L. But the mono-digestion appeared instability when FO concentration was 50 g/L. For the co-digestions of FW + FO, TS and VS reductions reached 70.7-86.1% and 87.5-91.4%, respectively, when FO concentration was 5-30 g/L. However, the inhibition occurred when FO concentrations increased to 40-50 g/L. The maximal FO loads of 40 g/L and 30 g/L were hence suggested for efficient mono-digestions and co-digestions of FO and FO + FW. (C) 2015 Elsevier Ltd. All rights reserved.