Abstract: The assembled aluminium internal floating-roof tank is of new structure which boasts of such characteristics as being safe in construction, easy to handle, fast to set up, good in corrosion protection and so on. The oil evaporation loss rate in this kind of tanks is determined by measuring gas at the time of major breath or making calculation using the empirical data accumulated in refineries, The measured results show that the loss reduction rate is 96.3% as compared with an arc-vault tank of the same capacity. The practical data of a 5, 000 m³ gassoline tank in Shanghai Refinery shows a that evaporation loss can be reduced by 187.88 tons, amounting to 124, 000 yuan in RMB. Besides economization of energy, the author claims, it is satistory for less enviroment pollution and greater safety in operation, for which it should be widely advocated and popularized.