Optical absorption and emission properties of a monosubstituted polyacetylene poly[(4-hexylcarbonyl-biphenyl-4'-oxy)-1-propyne] have been investigated by means of ultraviolet-visible spectrophotometry, fluorescence spectroscopy, and electronic structure calculations, respectively. Under ultraviolet excitations, dilute tetrahydrofuran solutions of the side-chain polymer can give off deep-blue photoluminescence. Our results have demonstrated that the optical properties of the liquid crystalline polymer are both concentration and temperature dependent. The origin of the deep-blue photoluminescence of polymer can be ascribed to the biphenyl chromophors in its pendants. Comparative studies to a biphenyl-containing liquid crystal give further evidence to our discussion on the origin of the deep-blue photoluminescence of the polymer.