The dependence of the low-temperature (5-150 K) photoluminescence spectra of poly(di-n-hexylsilane) (PDHS) incorporated into mesoporous silicas oil sizes and a geometry of pores was investigated. It was shown that visible luminescence in nanosized PDHS is observed only when the polymer is incorporated into the hexagonal pores of 2.8 nm in diameter, but, it is absent for the bulk film, dissolved polymer, and polymer incorporated into cubic pores of 2.8 nm in diameter or into hexagonal pores of 5.8 nm in diameter. We suggest that the appearance of visible luminescence in nanosized PDHS is connected with the transition from, electronic states localized on the conformational defect of the polymer chain.