Thermal stability of Pb(Zn1/3Nb2/3O3 (PZN) single crystal was examined in the saturated PbO vapor at 1150 degrees C for 2h and found that PZN single crystal decomposes into pyrochlore type compound, PbO and ZnO, which indicating that PZN is unstable at high temperature. The reason why PZN single crystal can be grown in the PbO flux at temperature higher than the decomposition into pyrochlore was explained by applying the Ostwald’s step rule. The reason PZN can be stabilized by partial substitution of A or B site cations was considered on the basis of the decrease of steric hindrance of lone pair of Pb2+ in the perovskite lattice and decrease of mutual interactions between lone pair and Zn2+. The result of electrostatic potential calculation for each cation and anion in Pb(B’B")O-3 showed that the site potential ratio, phi(B’)/phi(B") (the valence of B’ is lower than that of B") is much smaller in A(2+)(B1/32+B2/35+)O-3 and A(2+)(B1/22+B1/26+)O-3 than in A(2+)(B1/23+B1/25+)O-3 and A(2+)(B2/33+B1/36+)O-3 perovskite type compounds. The result suggested that in A(2+)(B1/32+B2/35+)O-3 and A(2+)(B1/22+B1/26+)O-3 type perovskite type compounds, large electronegativity difference from oxygen is required for B’ cation to decrease interaction with lone pair of Pb2+ in addition to the geometrical limit of the ion radius prescribed by the tolerance factor.