The hepatitis B virus (HBV) core protein has been found in the nucleus, the cytoplasm, or both of HBV-infected hepatocytes. However, the mechanism that regulates the subcellular localization of the HBV core protein is still unclear. In this report, we demonstrate that nuclear localization of the HBV core protein is cell cycle-regulated in two different cell lines. The amount of the core protein in the nucleus was increased during the G1 phase, reduced to an undetectable level during the S phase, and increased again when the cells were confluent and ceased to grow. Thus, the nuclear localization of the core protein during HBV infection can be at least partially attributed to liver injury and regeneration, which cause the hepatocytes to enter cell cycles. Based on the observation that the cytoplasmic core protein was phosphorylated and the nuclear core protein was not, we speculate that nuclear localization of the HBV core protein is negatively regulated by phosphorylation during the cell cycle.