Methane (CH4) production from palm oil mill effluent (POME) pre-treated by ozonation was conducted under mesophilic (37 degrees C) condition. The results demonstrated that methane can be produced from both non-ozonated and ozonated POME at a concentration range of 3,000 to 15,000 mg COD L-1. Methane yield rised 54% when POME was pre-treated by ozonation at POME concentration of 15,000 mg COD L-1. The methane yield increased the POME concentration was increased. At POME above 15,000 mg COD L-1, the methane yield was dropped dramatically. The methane production rates (R-max) and yields exerted similar trend regarding the POME concentration. Accumulation of volatile fatty acids in the reactor posed the drop of methane production. Ozonation pretreatment process of POME can improve the biodegradability of the complex organic matter in POME and enhanced methane yield and rate at POME concentration range of 3,000-15,000 mg COD L-1. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.