The radical anion of octa-tert-butyloctasilacubane was generated and isolated. The EPR spectrum showed the satellites due to the tertiary C-13 nuclei of the eight tert-butyl groups. The X-ray crystallographic analysis showed that the SiSi bonds are shortened and the SiC bonds are elongated compared with those of octa-tert-butyloctasilacubane. These results are well explained by the distribution of an unpaired electron in the singly occupied molecular orbital (SOMO).