Abstract: In this paper, the authors analyze the problems existed in modifying arch-roof tank into inner floating roof tank, point out that the perpendicularity and circularity of tank are the two most important parameters in the modification. The total technical requirements are that the overlap width of sealing material (for instance fluoro-elastomer, available to pure benzene) with tank shell should be 50-70 mm, the perpendicularity of tank shell opposite to tank bottom is not more than 5‰, the ellipticity of tank shell is not more than 30 mm. The authors consider that if these requirements are met, the evaporation loss can be greatly decreased for the improved tank.