An optimal control model on temperature control for quality of perishable foods is presented in this paper, in which the quality deteriorates according to the first-order reaction and the temperature is defined as a control variable. The objective functional includes the loss induced by quality deterioration and the cost of temperature control. To minimise the total cost, the optimal temperature is obtained by solving the optimal control problem with Pontryagin's maximum principle. It is strictly proved that the isotherm condition of storage is optimal, which can be implemented conveniently in practice and improve the economic benefits of enterprises. A numerical example is given to illustrate the effectiveness of the proposed method