Abstarct: There are 40 storage tanks in the Khark Island for storing crude oil at the ambient temperature for some periods of time to stabilize the flow between production wells and transporting ships. Considering the hot summers of the island, light hydrocarbons vaporize and accumulate in the space between the liquid and the tank roof, which are often vented to the atmosphere. This process causes environmental pollution and also affects the quality of the crude oil besides the economic detriment. Therefore, crude oil evaporation loss associated with the storage tank is an important issue which should be carefully investigated to identify the potential means of its reduction. The aim of the present work is to determine the evaporation rates from external floating storage tanks and to study the effects of their exterior surface paint on the losses due to the solar irradiation. The API standards along with the thermal analysis of the tank have been employed to evaluate the tank temperature and the evaporation losses of a typical tank baxsed on the actual ambient conditions of the Khark Island for one year. In this study a numerical scheme has been developed for estimating the time variations of the storage tank temperature and evaporation losses. The scheme is validated against the measured values of the storage tank temperature at different times during a day, where reasonable agreements are observed. Furthermore, the numerical value of monthly averaged evaporation losses have also been compared with the estimations baxsed on the API AP-42 standard. The results show that the absorptivity of the exterior surface paint has considerable effects on tank temperature variations and the evaporative losses accordingly. Paints with low absorption can reduce the evaporation losses significantly, when strong solar radiation is present. Furthermore, experimental data has been provided to validate the calculated tank temperature variations, where reasonable agreements have been found.