In order to characterize the functions of the sweetpotato SRF1 gene, which encodes a Dof zinc finger transcriptional factor preferentially expressed in the storage roots, we isolated its full length cDNA and produced transgenic sweetpotato plants with altered SRF1 expression levels. The isolated cDNA of SRF1 encoded a polypeptide of 497 amino acids and was closely related to the cyclic Dof factors of Arabidopsis and the ascorbate oxidase binding protein of pumpkin. SRF1 was most highly expressed in storage roots, although some expression was also observed in other vegetative tissue. Transgenic plants overexpressing SRF1 showed significantly higher storage root dry matter content compared to the original cultivar Kokei No. 14 or control transgenic plants. In these plants, the starch content per fresh weight of the storage roots was also higher than that of the wild-type plants, while the glucose and fructose content drastically decreased. Among the enzymes involved in the sugar metabolism, soluble acid invertase showed a decreased activity in the transgenic plants. Gene expression analysis showed that the expression of Ibbetafruct2, which encodes an isoform of vacuolar invertase, was suppressed in the transgenic plants overexpressing the SRF1 gene. These data suggest that SRF1 modulates the carbohydrate metabolism in the storage roots through negative regulation of a vacuolar invertase gene.