The localization mechanism for the excess electron and positive hole in the polysilane radical anion and cation has been investigated by means of both molecular dynamics (MD) and extended Huckel molecular orbital (EHMO) calculations. A linear oligosilane (SinH2n+2) (n = 32 and 64) was chosen as the model of polysilane. The geometry optimization of the polysilane at the MM2 level gave a regular all-trans form as the most stable structure. The MD calculations, started from the optimized structure, showed that the conformation of the polysilane skeleton was gradually randomized as a function of time by thermal activation at 300 K. The conformations at 0.0, 0.05, 1.0, 1.5, and 2.0 ps were chosen as sampling points, and spin densities on the silicon atoms were calculated by the EHMO calculations at each sampling point. An excess electron and a positive hole were fully delocalized along the Si chain in the regular all-trans form (time = 0.0 ps), whereas in the disordered conformations (time not equal 0) both electron and hole were completely localized on a few Si atoms (10-20 monomer units). The present calculations suggested that a continuous disorder of the Si main chain randomized by thermal energy (i.e., disordered dihedral angle of Si skeletons) is dominant in both electron and hole localization. The localization mechanism was discussed on the basis of theoretical results.