Hepatitis B virus (HBV) infection is associated with a broad spectrum of clinical manifestations, including cirrhosis and hepatocellular carcinoma (HCC). Endoplasmic reticulum (ER) stress and subsequent oxidative stress have been implicated in liver carcinogenesis and disease progression with chronic inflammation. In our previous study, several mutations in the precore/core region of HBV genotype C were identified from 70 Korean chronic patients, and the mutations were associated with HCC and/or HBV e antigen serostatus. Here, we found that the naturally occurring mutations P5T/H/L of the HBV core antigen induced ER stress. The upregulation of ER stress resulted in higher reactive oxygen species production, intracellular calcium concentration, inflammatory cytokines as well as surface antigen production and apoptosis of cells. This study suggested that these mutations may contribute to the progression of liver disease in chronic patients.