Histamine is a biogenic amine, which can cause food poisoning when present at high concentrations (>500 ppm). In situations where the formation of histamine in food cannot be prevented through traditional methods such as refrigeration, diamine oxidase (DAO) enzyme may be a suitable method to reduce histamine concentration to safe levels. The aim of this work was to apply the enzyme to cooked tuna soup, which is one of the manufacturing steps of Rihaakuru, which often contains high levels of histamine. The DAO activity in tuna soup containing 500 ppm of histamine, at various pH values (5-7) and salt concentrations (1-5%) were examined. A central composite design (CCD) was used which contained a total of fifteen experiments. Histamine was completely degraded (L = 0) at pH 7 and 6.5, and at salt level of 2 and 3%. The rate of histamine reduction was optimum (r > 5) at pH 7 and salt level 3%. To obtain complete histamine degradation and optimum rate of degradation simultaneously, salt 3% and pH 7 was suggested.