Optimal condition for methane production was F:I of 2:1, C:N of 40:1 and TS of 16%. Highest methane yield of 223 m3 CH4 tonne−1VS was achieved from EFB. Dominant archaea in SS-AD were by Methanosaeta sp. and Methanoculleus sp. Dominant bacteria in SS-AD were Thermoanaerobacterium sp. and Clostridium sp. Oil palm biomass has great potential for methane production by thermophilic SS-AD.