The effect of operating parameters (temperature and time) and various reaction modes(batch, semi-batch and continuous flow) on the behavior of amino acid production from hydrothermal decomposition of fish-derived wastes was investigated. The amino acids obtained in batch experiments at 523 K were mainly alanine (Ala) and glycine (Gly) at maximum yield of 65 and 28 mg/g-dry fish, respectively. At a relatively lower temperature of 473 K, the relative concentration of high-molecular-weight amino acids such as aspartic acid (Asp) and serine (Ser) is significantly high, but decreases as temperature increases. It is likely that high-molecular-weight amino acids decompose faster than low-molecular ones. Semi-batch and continuous flow modes of reaction suppressed decomposition of amino acids into organic acids (or volatile materials) by continuously removing the products from the reaction zone as soon as they were formed. Thus, a large amount of high-molecular-weight amino acids such as Asp and Ser at these reaction modes was observed. This increases the yield of total amino acids at short reaction time and at temperature relatively higher than 523 K. It was also observed that the composition of the resulting products also depends on the modes of reaction.