The piezoelectric property of -oriented 0.5%MnO2-(K0.5Na0.5)NbO3 (Mn-KNN) crystals was studied as a function of domain size, being poled with different electric fields at 205 C (above orthorhombic to tetragonal phase transition temperature To-t). The piezoelectric coefficients d33 and relative dielectric constants er were found to increase from 270 pC/N to 350 pC/N and 730 to 850 with the domain size decreasing from 9 to 2 lm, respectively. The thermal stability of piezoelectric property was investigated, where the d33 value for -oriented Mn-KNN crystals with domain
size of 2 lm was found to decrease to 330 pC/N at depoling temperature of 150 C, with minimal variation of 6%. The results reveal that domain size engineering is an effective way to improve the piezoelectric properties of Mn-KNN crystals.