Patients, who are coinfected with hepatitis B and C, are at high risk for a progression to severe liver disease (cirrhosis, decompensated liver disease), hepatocellular carcinoma, and unresponsiveness to treatment. In this article, a case who developed a reactivation of chronic hepatitis B subsequent to Chronic Hepatitis C (CHC) treatment is presented. A 42-year-old woman with HBV and HCV was admitted to infectious diseases clinic with a major complaint of fatigue for three months. Laboratory tests revealed an elevated alanine transaminase of 182 IU/L (Normal range: 0-55 IU/L), elevated aspartate transaminase of 150 IU/L, serum HCV-RNA levels of 2080195 IU/mL (genotype 1b), and HBV-DNA of 1184 IU/mL. The results of other laboratory tests were within the normal range and remained to be normal during the follow-up. Pegylated interferon alpha-2a (180 mcg/week) and ribavirin (1200 mg/day) were administered for the treatment of CHC for 48 weeks. After one year of CHC treatment, our case was admitted with the elevation in liver enzymes and HBV-DNA of 878215 IU/mL. The histopathological examination of liver tissue revealed a mildly active histologic activity (6/18) and a mildly active histological activity (1/6) according to ISHAK scoring. Tenofovir disoproxil tablet (245 mg/day) was initiated for the chronic hepatitis B treatment. The patient is followed up at the ambulatory clinic of infectious diseases. As a result, patients coinfected with HBV and HCV should be followed up for the reactivation of HBV after CHC treatment with either pegylated interferon-based therapy or oral direct-acting antiviral agents in the medium-term, and the cirrhosis and hepatocellular carcinoma in the long-term.