Abstract: The paper presents the old and new formulas about the evaporation loss in small breathing type of fixed roof tank recommanded by American Petroleum Institute (API). According to different kinds of oil, the evaporation loss is calculated with the old and new formulas and the experiment results are compared and analyzed. The applicable scope and accuracy of the old and new formulas are given and their sensitivities are analyzed. Research shows that the new formula from API has wider applicable scope than the old one. The calculated error in evaporation loss by old formula for gasoline is a bit larger than the new one. However, for oil and glycol the error is much larger. With the new developed TANKS 4.09D software and integrated database, the new formula is more convenient to use. Sensitivity analysis on the two formulas under the respective parameters is performed and the dependence of the two formulas on the respective parameters is summarized.