Entecavir, a potent antiviral agent, has been widely used for treatment-naïve chronic hepatitis B patients. However, about 20% of patients showed partial virologic response after 2 year of entecavir therapy (33% in HBeAg positive, 10% in HBeAg negative patients). Tenofovir is a nucleotide analogue with more potent antiviral activity. In addition, there is no cross resistance between the two drugs. Therefore it is assumed that tenofovir would be effective in the treatment of chronic hepatitis B patients who shows partial virologic response (detectable HBV DNA by real time PCR after 12 months of treatment) despite treatment with entecavir. In this study, we will compare the efficacy of switching to tenofovir with continuing entecavir in patients who shows partial virologic response to entecavir.