This study explored co-cultivation of Nannochloropsis oculata microalgae with oil palm empty fruit bunch (OPEFB) for anaerobic biomethane production and palm oil mill effluent (POME) treatment. The highest specific biogas production rate (1.13�1.14 m3 kg�1 COD d�1) and methane yield (4606�5018 mL CH4 L�1 POME d�1) were achieved with co-cultivation of N. oculata (2 mL/mL POME) and OPEFB (0.12 g/mL POME), as similarly predicted by response surface methodology for optimum conditions. Without microalgae and OPEFB co-cultivation, the biomethane yield was 1.3-fold lower, although the specific biogas production rate remained constant at 1.13�1.16 m3 kg�1 COD d�1. Aerobic and anaerobic treatment of POME after 7 d with microalgae achieved higher removal efficiency of COD (90�97), BOD (84�98) and TOC (65�80) as compared to without microalgae with COD (58�68), BOD (77�86) and TOC (58�68). © 2014 Balaban Desalination Publications. All rights reserved.