Study on size effects of corn stalks for biogas production was processed with three different sizes of corn stalk: 15 – 20 cm, 8 – 10 cm and 1 – 2 cm. The 21 L anaerobic digesters were set up to implement experiments in lab conditions. All experiments were fed with loading ratio of 1 kg volatile solids (VS)/L*d^-1, the hydraulic retention time was 60 days. The recorded parameters from all experiments (temperature, pH, alkalinity) did not significant different between treatment but ranged in suitable values for anaerobic fermentation process. The biogas yield and CH₄ concentrations got highest value at treatment with sizes 8 – 10 cm, the next was 1 – 2 cm and the lowest was 15 – 20 cm with 526 L/kgVS - 38.3%, 440.5 L/kgVS - 29.2% and 312.2 L/kgVS - 26.4% in respectively. The %CH₄ of all treatments tended to increase by day and kept more stable in treatment with size 8 – 10 cm than the rest. The results show that using corn stalks in size 8 – 10 cm produced more biogas than other sizes. However, co-digestion corn stalks with other nitrogen riches materials have better biogas production due to high C/N ratio of corn stalks.