We provide an analysis of "on-surface polymerization" for a diacetylene (DA) compound 10,12-pentacosadiyn-1- ol (PCDYol) by scanning probe microscopy. For monomolecular layers of the PCDYol on graphite, it is known that two different two-dimensional polymorphic forms—herringbone (H) and parallel (P) arrangements—are observable at room temperature. The chain polymerization of the PCDYol forms the conjugated polydiacetylene (PDA), which is expected to work as the nanowire for the molecular devices. Here, we study the thermodynamic stability of these polymorphs. As a result, we find that the P arrangement is the stable structure while the H arrangement is the quasi-stable one. We also show that the melting point of the P arrangement is 20 _oC higher than that of the bulk crystal probably due to the epitaxial alignment of the PCDYol to the graphite lattice. Furthermore, we statistically analyze the polymerization degree of the PDA chains formed by UV irradiation at different temperatures in air. The distributions of the polymerization degree agree well with the prediction from a simple probabilistic model, allowing for addition reaction and deactivation at both the ends of the chain as stochastic events. The estimated activation energies of the addition reaction and deactivation are noticeably different from those of the conventional solid-state polymerization in the bulk crystals of the DA. Thus, we conclude that the graphite surface and O₂ gas in air atmosphere drastically affect on the photo-induced chain polymerization of the PCDYol. Keywords: conductive polymer,