The behavior of protein decomposition to amino acids in near-critical water is elucidated by using silk fibroin as a model compound. Results show that serine (Ser), aspartic acid (Asp) and other complex amino acids, obtained initially in significant amount, gradually decreased as reaction time and temperature were increased. At temperature higher than 523 K, it is likely that Gly and Ala underwent further decomposition. Regarding the effect of various additives, no significant effect on the yield of amino acids was observed with the addition of oleic acid. However, the presence of NaOH and formic acid (FA), both in 5 mol% aqueous solution, had significant effect on the yield. The effect of alkalinity and acidity, the production of amino acids was enhanced in either acidic or basic conditions.