The present invention relates to a modified CpG oligodeoxynucleotide (ODN) which is prepared by coupling a consecutive sequence of deoxyribothymine (dT) to the 3'-terminus of CpG ODN having immunoregularory function, thereby improving immunoactivity of splenocytes, macrophages and peripheral mononuclear cells, and therefore, can be effectively used as a vaccine adjuvant for preventing and treating hepatitis B or an anti-cancer agent. Since the phosphorothioate CpG ODN having the consecutive sequence of dT at its 3'-terminus shows high activity inducing Th-1 immune response and does not elicit in vivo toxicity with guaranteeing its safety, it can be effectively used as a vaccine adjuvant.