It is important either for clinical potential or mechanism elucidation to study abnormal efficient radiosensitizer. Various 4-nitroimidazoles 5-substituted by sulphonamide, sulphonate or thioether groups have shown much more efficient radiosensitization effect than that predicted by their one electron reduction potential (,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767578&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767580&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42767579&type=,). Accordingly, Some of them have been calculated by MO-CNDO/2 method. The calculated results were summarized as follows:,In nitroimidazoles, the positive charge concentrates on nitrogen atom of the nitro group, but there are two positive charge centers in these ortho-substituted 4-nitroimidazales. One of them located on nitrogen atom of the nitro group, and the other located on sulphur atom of the 5-substituted group. The positive charge on sulphur atom is more than that on nitrogen atom.,Another character of abnormal radiosensitizers is that their three frontier empty orbital energies are all lower than the lowest empty orbital energy of the target molecules, thymine. For nitroimidazoles, only their lowest empty orbital energies are lower than that of the target molecules. For imidazoles 5-substituted by sulphonamide, sulpohnate or thioether groups, their two frontier empty orbital energies are lower than the lowest empty orbital energy of the target molecules.,The electron density distributions of three frontier empty orbit of calculated abnormal radiosensitizers were given and it was shown that their electron densities concentrates on 5-substituted group for both the first (ie the lowest empty orbit) and the third frontier empty orbit, but the electron densities for the second frontier empty orbit concentrates on nitro group and imidazole ring. Therefor the combination of the 5-substituted sulfur containig groups, and the nitro group induced the much more efficient radiosensitive effect of the ortho-substituted 4-nitroimidazoles. In these compounds, 5-substituted sulfur containing groups may be more impartant than nitro groups for their abnormal radiosensitization.