The surface growth mechanism of the primary soot particles was experimentally examined in the post flame region of an atmospheric CH4/O-2 premixed flame. The size distributions of the primary soot particles were measured by using a diffusion battery connected with a condensation nuclear counter. Some polycyclic aromatic hydrocarbons in the post flame region were measured by capillary gas chromatography/mass spectrometer. The size of the primary soot particles gradually became uniform, accompanying an increase of the mean particle size in the post flame zone. These results show that the small particles are more reactive than the large ones. The phenanthrene concentration had a much steeper decrease than that of other polycyclic aromatic hydrocarbons observed in the post flame zone. The injection of phenanthrene in the post flame region increased the soot particle size, while that of C2H2 did not influence the soot particle size at all.