Nucleation and growth mechanism of polypyrrole (PPy) films was investigated on vapor deposited gold/highly oriented pyrolytic graphite (Au/HOPG) after heat treatment by means of current–time transient (i–t) measurements and tapping mode atomic force microscopy. It was found that the nucleation and growth was a progressive 3-D before and after nuclei overlapping. However, the mechanism was 2-D instantaneous before nuclei overlapping and 3-D progressive after nuclei overlapping on a Au/HOPG substrate without heat treatment. The change in mechanism was due to lower surface defects on a heat-treated Au/HOPG substrate than on a bare Au/HOPG substrate. Images of PPy on a heat-treated Au/HOPG contained zig-zag and nodular structures.