Pyrolysis of a polyacetylene thin film has been performed in order to carbonize at temperatures of 500 to 1000 °C in vacuum. A trans-polyacetylene thin film was synthesized using a Ziegler–Natta catalyst. A black char below 20% in weight of the original PA film remained after pyrolysis. Structural properties and morphology of the black chars were investigated using Raman scattering spectrum, X-ray diffraction measurements, and scanning and transmission electron microscopy. Dehydrogenation and carbonization of the PA film were almost finished at a pyrolysis temperature of 800 °C. However, hollow spherical or elliptical nano-particles of tens of nanometers in size, which are composed of graphite structure, were included in the black chars obtained at all pyrolysis temperatures in this work. The formation mechanism of a graphite crystal in nanometer size from a PA crystal was discussed.