This study investigated the effect of excess PbO and post-annealing on the crystallinity and the microstructure of Pb(Mg_1/3Nb_2/3)O₃ (PMN) epitaxial thin films. PMN thin films were deposited on SrTiO₃ (STO) (001) substrates using a chemical solution deposition (CSD) process with rapid thermal annealing (RTA) of spin-coated metallo-organic decomposition (MOD) solutions. The nominal film composition was Pbx(Mg_1/3Nb_2/3)O₃ (x = 1.0 or 1.1). All films showed the (001)_PMN//(001)_STO, [001]_PMN//[001]_STO epitaxial relationship and were single phase perovskite PMN without any pyrochlore phases. The Pb_1.1(Mg_1/3Nb_2/3)O₃/SrTiO₃ thin film post-annealed at 973 K with facing treatment showed the highest crystallinity of 0.8° in full width at half maximum (FWHM) of the 002_PMN rocking curve. Transmission electron microscopy (TEM) revealed PbO nanoparticles on the surface of the PMN thin films. The film composition was almost stoichiometric. On the other hand, the Pb_1.1(Mg_1/3Nb_2/3)O₃/SrTiO₃ thin film post-annealed at 973 K without facing treatment showed crystallinity of 1.0° and no PbO nanoparticles. The film composition was almost stoichiometric. These results indicate that the excess PbO is not incorporated into the PMN films. Instead, it volatizes from the film surface, which improves the crystallinity and inhibits the PMN decomposition.