<P><B>Abstract</B></P> <P>Palm oil mill industry generates a large amount of palm oil mill effluent (POME) and empty fruit bunches (EFB) as byproducts. Biogas production from anaerobic co-digestion of POME with EFB was investigated under mesophilic condition. The amount of POME and EFB was varied from 2 to 10 gVS and size of EFB was varied from 0.5-6 centimeter. High methane yield and high biodegradability were achieved at the size of 0.2 mm with 63-70% biodegradability with a methane yield of 320 mL CH<SUB>4</SUB>/gVS. The POME:EFB ratio of 4.5-7.5 and size EFB of 3.3-6 centimeters shown acceptable biodegradability value of 40-50% with a methane yield of 282 mL CH<SUB>4</SUB>/gVS. The POME:EFB ratio of 4.5-7.5 and size EFB of 3.3-6 centimeters was more practical for biogas production due to reducing the cost of size reduction</P>