Abstract: In the present paper, CNDO—crystal orbital method was applied to calculate the electronic structures of polypropynenitrile（PPN） and other substituted polyacetylenes. The steric effects of substituents of cyano, amino, chloro, fluro, methyl, and phenyl groups, their electron-with-drawing or electron-donating abilities and Conjugate effects with main chain of polymers were evaluated. The emphasis was put on the investigations of steric effect of cyano group in PPN, its strongly electron-withdrawing ability and the influences on energy-band structures, Based on the calculated results and our experimental facts, the structure-property relationship of PPN can bc better interpreted.