Electromechanical property measurements and microstructure observations using optical microscopy were performed on a [001](c) oriented k(33) resonator made of 91%Pb(Zn1/3Nb2/3)O-3-9%PbTiO3 single crystal, which was polarized under different electric fields. At room temperature, when the poling field is 1100 V/mm, the electromechanical coupling factor k(33) is 0.90 and piezoelectric coefficient d(33) is 1665 pC/N. Such superior electromechanical properties could be attributed to the formation of monoclinic multi-domain structure, which transforms to tetragonal phase at 46 degrees C. While at a higher poling field of 1200 V/mm, the crystal becomes single-domain tetragonal state and its k(33) and d(33) are only about 0.69 and 850 pC/N, respectively. The critical poling field to transform the monoclinic phase to the tetragonal phase is found to be similar to 1120 V/mm.