Unmethylated CG-rich oligodeoxynucletides are exhibiting several immunological effects. The backbone and the flanking sequences of CpG-ODN decide the potence and specificity of its immunostimulatory effects. Chemical modifications have been systematically in corporated into CpG-ODN and the resulting modified compounds have been studied for immunostimulatory activity,the researches show that they have strong effect on species and cells'speciality, which offer the instruction for the design of CpG-ODN.