The helical structure is experimentally determined by circular dichroism (CD) spectra. The sign and shape of the CD spectra are different between B-DNA with a right-handed double-helical structure and 2 Z-DNA with a left-handed double-helical structure. In particular, the sign at around 295 nm in CD spectra is positive for B-DNA, which is opposite to that of Z-DNA. However, it is difficult to determine the helical structure from the UV absorption spectra. Three important factors that affect the CD spectra of DNA are (1) the conformation of dG monomer, (2) the hydrogen-bonding interaction between two helices, and (3) the stacking interaction between nucleic acid bases. We calculated the CD spectra of (1) the. dG monomer at different conformations, (2) the composite of dG and dC monomers, (3) two dimer models that simulate separately the hydrogen-bonding interaction and the stacking interaction, and (4) the tetramer model that includes both hydrogen-bonding and stacking interactions simultaneously. The helical structure of DNA can be clarified by a comparison of the experimental and SAC-CI theoretical CD spectra of DNA and that the sign at around 295 nm of the CD spectra of Z-DNA reflects from the strong stacking interaction characteristic of its helical structure.