Abstract: If the oil vapor from the tanks is discharged into the atmosphere, the environment pollution will be caused and the human health will be threated. Definitely, pressure and temperature changes are the main factors causing evaporation of oil products in the tank. As for the breathing loss of oil tanks, the formula concerning the minimal reduction rate of the breathing loss of pressure tanks was derived based on the API theoretical formula, so as to reduce the breathing loss, even to completely eliminate the small breathing loss, by controlling the pressure with a breather valve of oil tank. According to the analysis on influence of temperature change on expansion factor of monomer hydrocarbon (propane, n-butane, isobutene, n-pentane and n-hexane) and various oil products (93# gasoline, 97# gasoline, naphtha, kerosene, diesel oil and Panyu crude oil), the results show that when the storage pressure of the tanks is designed for oil products of different types and compositions, tank with different design pressure shall be adopted. In addition, appropriate type of breather valve can be used, or the tank wall can be thickened to increase the control factor, further to reduce the breathing loss of tank. The research results could be used as theoretical reference for oil tank design and oil depot management.