The Mekong Delta - a “rice bowl” of Vietnam - discharges around 30 million tons of paddy straw annually but most of straw is in situ burning that causes loss of precious nutrients and polluting the environment. Meanwhile, anaerobic technology was introduce in the region for long time but not yet become popular due to lack of input materials. The lab-scale 20 L anaerobic digesters were applied to co-ferment paddy straw (with sizes of 1.0 cm, 10.0 cm, 20.0 cm, original size) and pig manure (mixing ratio of 50%:50% based on their organic dry matter values) in 60 continuous days. The recorded values of temperature (25.3 to 30.3⁰C), pH (6.74 ± 0.18), and redox potential (-316.4 to -128.2 mV) of almost treatments were suitable for activities of methanogenic micro-organisms. The biogas yield of treatment with straw size of 1.0 cm was 691.05 L/kg ODM_fermented, of 10.0 cm - 687.79 L/kg ODM_fermented, of 20.0 cm - 685.08 L/kg ODM_fermented and original size - 680.44 L/kg ODM_fermented which significant different to the treatment of 100% pig manure. The results confirmed rice straw could apply as additional material to the pig manure at anaerobic digester to produce biogas that eliminate in situ straw burning.