A resource recovery technique using sub- and supercritical water hydrolysis was applied to convert waste fish entrails into amino acids. The effect of reaction parameters such as temperature and time necessary for the control of reaction towards optimum yield of amino acids was investigated. Results showed a maximum yield of total amino acids (137 mg/g dry fish) from waste fish entrails at T = 523 K (P = 4 MPa) and reaction time of 60 min in a batch reactor. Under supercritical conditions (e.g., T = 653 K, P = 45 MPa), the yield decreases due to rapid decomposition compared to production rate of amino acids. The results suggest operation of the system at short reaction time and mild temperature condition.