Thin films of 1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD) grown epitaxially on (001) surface of KCl through vacuum-deposition were examined on their polymerisation process induced by heat treatment or electron irradiation. The structural changes due to polymerisation in the films were investigated by electron diffraction and low temperature high resolution electron microscopy. By electron irradiation, many small crystallites of polymer were produced randomly in the monomer single crystal keeping the topochemical relation. The nucleation of polymer crystals is considered to be ＇homogeneous＇ in this case. In contrast, thermal polymerisation resulted in bigger fibrous polymer crystals with the same topochemical relation, but different orientations were also observed at the edges of the crystal, which means that the polymerisation is ＇heterogeneous＇. High-resolution images of partially polymerised specimens by heat treatment showed small grains of transient states with intermediate lattice spacings between monomer and polymer crystals. Therefore, the polymerisation of DCHD proceeds through the transient stage of crystal structure without definite phase separation of monomer and polymer in the case of thermal polymerisation. In addition to this, the polymer crystals were found to be formed first at the edges of monomer crystals, and thereafter may change their orientation so as ts relax an inner stress produced by further polymerisation at the inner part of monomer crystals.