Objective: To study the durability of the anti-demineralization effects of fluoride varnish after being applied to dental root surfaces. Methods: Coronal and radicular dentin samples were prepared from extracted human teeth. Duraphat^® (DP) was applied to the dentine surfaces to form a protective film. The film-dentin interfaces were observed by scanning electron microscopy (SEM) and the fluoride element was analyzed with energy dispersive spectrometer (EDS). Thus, the differences between applying DP on crowns and roots were compared. Radicular dentin samples were prepared and randomly divi-ded into four groups: (1) Blank: DP was not applied, and demineralized in acetic acid (pH 4.5) for 4 days; (2) Blank+aging: DP was not applied, the samples were put into deionized water for 14 days at room temperature, and then demineralized in acetic acid (pH 4.5) for 4 days; (3) DP: DP was applied and demineralized in acetic acid (pH 4.5) for 4 days; (4) DP+aging: DP was applied, the samples were put into deionized water for 14 days at room temperature, and then demineralized in acetic acid (pH 4.5) for 4 days. Finally, SEM observation and EDS analysis of fluoride content were performed on film-dentin interfaces to evaluate the degree of demineralization, the morphology of DP film, and the penetration of fluorine. Results: The immediate penetration depth of fluoride element from DP was deeper in the coronal dentin than that in radicular dentin. The samples in the blank and blank+aging groups demine-ralized significantly after acid etching. The DP group did not undergo demineralization, and the fluorine element penetrated to (76.00±8.94) μm below the interfaces. The structure of the protective film in the DP+aging group was damaged, but the underneath dentin did not undergo demineralization. The fluorine element still remained at a depth of (5.00±3.53) μm below the interfaces. Conclusion: DP has an anti-demineralization effect on the root surface, and this effect can still be exerted for a period of time after losing the structure of protective film. It has the ability to prevent root caries and a certain durability.