The crystal structure of 1,4-benzenedithiol (BDT) was determined by the Rietveld method based on the calculation of the atomic coordinates of the BDT molecule using the Molecular Mechanics Program (MMP2). The refined crystal structure of BDT was monoclinic Pz,le with dimensions, a = 7.795, b = 7.290, c = 5.955 Angstrom, beta = 92.18 degrees, z = 2. The R factor of the refined structure was 0.038. Using above results, the mechanism of solid-state addition polymerization of BDT to 1,4-diethynylbenzene (DEB) was studied. Sublimed BDT piles up onto glass plate substrate and forms the layer structure along with the a axis. An inclination angle of the piled BDT column was 60 degrees toward the substrate surface. DEB crystal structure was also monoclinic P2(1)/c with a = 4.007, b = 6.018, c = 15.340 Angstrom, beta = 91.42 degrees, z = 2. Sublimation of equimolar mixture of BDT and DEB gave a crystal having 1 : 1 composition, in which DEB column is situated between the columns of BDT. Relative arrangement of both monomers was suitable for the addition of -SH and -C=CH groups, since the distance between the two groups is 3.3 Angstrom by CERIUS II calculation. Therefore, the addition polymerization of BDT to DEB easily proceeded by UV irradiation and the resulting polymer had a highly layer structure along with the a axis of BDT crystal. Tentatively estimated crystal structure of polymer obtained is monoclinic with a = 7.73, b = 7.30, c = 5.95 Angstrom, beta = 92.16 degrees.