By controlling the temperature of chemical reaction, we synthesized four polyphenylacetylene samples, in which the percentages of cis configurations were determined to be 65, 75, 85 and 100%, respectively. Ultraviolet-visible and fluorescence studies show that the cis-configured backbones have posed significant effects on the optical absorption and fluorescence of the polyphenylacetylenes. Upon the 325-nm excitation, the dilute solutions of these cis-polyphenylacetylenes give off weak fluorescence with their peak located at about 390 nm, but the concentrated solutions of the polymer can give off strong orange-red fluorescence with their peak located at about 600 nm. With Hückel tight binding theory, both the E-k dispersion relations and the density of states were calculated for cis-polyphenylacetylene. Our results suggest that the blue photoluminescence of polyacetylenes origins from the backbone enhanced -conjugation of the phenyl chromophors in the polymer, and that intermolecular interactions can occur in the excited complex of the polymer.