**5. Summary**

Roles of domain and grain boundaries in relaxor single-crystal plates and ceramics were clarified by measuring sound velocities. Effect of domain boundaries on elastic constants was evaluated by the effect of DC poling on elastic constants in (100)0.70Pb(Mg1/3Nb2/3)O3– 0.30PbTiO3 relaxor single-crystal plates. Effect of grain boundary on elastic constants was evaluated by comparing elastic constant in the relaxor single crystals and the ceramics. Finally, the different origins of piezoelectricity in single crystals and ceramics were proposed through our investigation of the measurement on sound velocities.
