Chain conformations due to thermal isomerization in poly(phenylacetylene-d(1))(PPA-d(1)) were investigated by H-2 solid State NMR. The observed H-2 NMR line shapes at -100 degrees C for PPA-d(1) and an annealed sample, An-PPA-d(1), consisted of both large and small Fake doublet peaks. The former doublet peak was attributed to the static segments in the polymer chain. Deuterium nuclear quadrupole coupling constants of the large doublet peaks for PPA-d(1) and An-PPA-d(1) were 164 and 168 kHz, respectively. A quadrupole coupling constant and the formation energy of various chain conformations were calculated using semiempirical molecular orbital methods (AM1 and PM3 Hamiltonians). Agreement between theory and experiment confirmed that an exothermic peak at 177 degrees C in the DSC thermogram for PPA-d(1) is due to the cis-trans isomerization from the cis-transoidal form to 80 degrees-deflected trans-transoidal one.