This study examined improvement of the crystallinity and the decomposition mechanism for the Pb(Mg_1/3Nb_2/3)O₃ (PMN) thin films using chemical solution deposition (CSD) technique with a metallo-organic decomposition (MOD) solution. The PMN thin film crystallinity increases with higher annealing temperature, but improvement is slight at temperatures higher than 700°C. The facing annealing has a distinct effect on the crystallinity of PMN thin film at 650°C, which is the lowest annealing temperature examined in this study. The ratio of the pyrochlore phase has a peak at 750°C with or without the facing annealing. The facing annealing also has a distinct effect on PMN thin film decomposition at 800°C. Improvement of the crystallinity and of the homogeneity of the morphology of PMN thin films deposited at 650°C was possible under the facing condition. The decomposition occurs locally, and the residual PMN region has high crystallinity and stoichiometry. The atomic structure of the PMN lattice has a fluctuation that is characteristic in the relaxor, which is related to the polar-nano region (PNR). The chemically ordered region (COR) also appears in PMN even at a lower annealing temperature than that of bulk relaxor materials.