An anaerobic digester system is a sequence of process to digest biodegradable waste into biogas in the absence of oxygen. In two-stage anaerobic digestion system acid-forming steps or hydrolysis stage are separated from the methane forming steps. Although hydrolysis stage tends to get too acidic, addition of alkali substance can prevent pH from dropping too low so as to maintain good decomposition condition for microorganism. Oyster shell powder is a useful pH control additive containing CaCO3 at high percentage that can neutralize acid. In this study, the performances between industry-made fine oyster shell (IOS) powder (size 10.5 µm) and manually ground oyster shell (OS) powder (size < 1 mm) in methane generation yield were compared. NaOH, which is an alkali reagent for controlling pH, also used in comparison. The result showed that at the end of the hydrolysis stage, IOS powder increased pH up to 6.63, NaOH did almost the same (6.72), and OS powder was the lowest (6.1). In liquid residue, ratio of inorganic ash content with IOS treatment was the highest (2.1 %), but OS was the lowest (1.4 %). In the methanogenesis stage, CH4 concentration with NaOH treatment was the highest (80 %) compared to oyster shell powders: 74.33 % in IOS and 74.24 % in OS. Average methane yield over observation period of IOS treatment was the highest (533.9 mL/gVS), followed by alkali (487.3 mL/gVS) and OS (413.7 mL/gVS). Total CH4 from IOS treatment was 37 % and 8 % higher than OS and alkali treatment. Powder size of oyster shell greatly affected pH control, methane yield, and solid-liquid separation, but not methane concentration. Using IOS powder as pH control in hydrolysis of two-stage anaerobic system resulted in 78 % less cost than using NaOH.