The effect of atmosphere on the chemical stability of Pb(Mg1/3Nb2/3)O-3 (PMN) ceramics at high temperature was examined. Heating the PMN pellet in air caused PbO vaporization to decompose PMN into a pyrochlore-type compound and Mg4Nb2O9 from surface toward the bulk. The decomposition rate was controlled by the Pb diffusion rate. The surface layer consisting of pyrochlore-type compound and Mg4Nb2O9 significantly lowered the dielectric constant. The PMN pellet after decomposition was heated again under PbO vapors produced by a powder mixture of PbO and ZrO2. Consequently, it was found that the pyrochlore-type compound and Mg4Nb2O9 reacted with PbO to form PMN from surface toward the bulk. This indicates that the decomposition reaction is reversible by the transportation of PbO from surface into the bulk.