The human interleukin-2 signal peptide and a potent universal helper T lymphocyte epitope PADRE were spliced to the 5' terminus of hepatitis B viru score HBcAg gene. The modified HBcAg gene was used to construct a DNA vaccine. After the resulted DNA vaccine construct was transfected into COS7 cells, secreted HBcAg was detected in the supernatant by ELISA. BALB/c mice were vaccinated intramuscularly with the modified HBcAg DNA vaccine and the wild-type one. Serum antibodies,T lymphocyte proliferative response and cytotoxic T lymphocyte response of the immunized mice were measured. The results showed that the modified DNA construct induced cellular and humoral immune responses much stronger in vivo than the natural one did, indicating the potential value as a therapeutic vaccine for treatment of chronic hepatitis B.