Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 [BNT] ceramics were prepared by controlling the quenching rate (QR). The quenching effects mainly caused increase in lattice distortion 90-α and T d, contribution of electrical properties and ordered structural phase transitions. In this study, we analyzed oxygen tracer diffusion and examined domain structures for quenched BNT ceramics, and then we proposed a mechanism for the quenching effect. As a result, the diffusion coefficients D of 18O tracer for the OF and quenched samples were 2.5×10-11 and 1.8×10-11 cm2/s, respectively, and there were no significant differences in oxygen vacancies after quenching. Thus, the correlation between the quench effect and oxygen vacancies was low. On the other hand, the quenching increased the domain size. Therefore, we propose that the domain structure is strongly correlated with the quench-induced increase in 90-α and T d, its contribution to electrical properties and ordered structural phase transitions.