The effects of nano-graphene on methanogenesis in anaerobic digestion was investigated. Short-term results showed that graphene (30 and 120 mg/L) had significantly positive effects on methane production rate, which increased by 17.0% and 51.4%. Further investigation indicated that acetate-consuming methanogenesis was enhanced. The failure of quinones to replicate graphene stimulation effects on methanogenesis suggested that graphene did not function as electron shuttles. After 55 day's operation at room temperature (from 20 to 10 degrees C, the methane production rate with 30 mg/L graphene was 14.3% higher than that of the control, while 120 mg/L graphene showed a slight inhibition on methane yield. Illumina sequencing data showed that the archaeal community structure remained fairly constant as the incubated sludge with graphene at low temperature, in which Methanoregula, Methanosaeta and Methanospirillum were the dominant species. Besides, Geobacter enrichment was observed with graphene, suggesting that the direct interspecies electron transfer might be promoted. (C) 2016 Elsevier Ltd. All rights reserved.