We previously found that hepatitis C virus (HCV) core protein, which possesses the consensus sequence of genotype 1b, transcriptionally activates the interferon (IFN)-inducible 2'-5'-oligoadenylate synthetase (2'-5'-OAS) gene in human hepatocyte cells. To clarify the mechanism of this activation, we further characterized the core protein as an activator of the 2'-5'-OAS gene. We demonstrated that the activation of the 2'-5'-OAS gene by the core protein is a general phenomenon, regardless of HCV genotype and strain. We showed that the 20 N-terminal amino acids (aa) of the core protein were important to the activation of the 2'-5'-OAS gene, although this N-terminal region did not have any effect on the subcellular localization of the core protein. We demonstrated that the core protein was able to activate all promoters possessing the IFN-stimulated response element (ISRE) examined. However, we found that the level of activation of the 2'-5'-OAS gene promoter possessing a particular variant type of ISRE was significantly higher than that of other IFN-inducible gene promoters. This phenomenon was confirmed using synthetic promoters possessing five repeats of the consensus or a 2'-5'-OAS-type ISRE. In addition, we showed that gene activation induced by the core protein is mediated by the ISRE. These results imply that the core protein prefers a subclass of IFN-inducible genes, the promoters of which possess the 2'-5'-OAS-type ISRE. Accordingly, we found that the IFN-inducible double-stranded RNA-specific adenosine deaminase gene promoter, possessing a 2'-5'-OAS-type ISRE sequence, was also efficiently activated by the core protein. The exact mechanism by which the core protein enhances gene expression was not determined, but we could find no effects of core protein on gene expression and phosphorylation status of the components of the JAK-STAT signaling transduction pathway.