Defect　engineering　is　a　well-established　approach　to　customize　the　functionalities　of　perovskite　oxides.　In　demanding　high-power　applications　of　piezoelectric　materials,　acceptor　doping　serves　as　the　state-of-the-art　hardening　approach,　but　inevitably　deteriorates　the　electromechanical　properties.　Here,　a　new　hardening　effect　associated　with　isolated　oxygen　vacancies　for　achieving　well-balanced　performances　is　proposed.　Guided　by　theoretical　design,　a　well-balanced　performance　of　mechanical　quality　factor　(Q(m))　and　piezoelectric　coefficient　(d(33))　is　achieved　in　lead-free　potassium　sodium　niobate　ceramics,　where　Q(m)　increases　by　over　60%　while　d(33)　remains　almost　unchanged.　By　atomic-scale　Z-contrast　imaging,　hysteresis　measurement,　and　quantitative　piezoresponse　force　microscopy　analysis,　it　is　revealed　that　the　improved　Q(m)　results　from　the　inhibition　of　both　extrinsic　and　intrinsic　losses　while　the　unchanged　d(33)　is　associated　with　the　polarization　contributions　being　retained.　More　encouragingly,　the　hardening　effect　shows　exceptional　stability　with　increasing　vibration　velocity,　offering　potential　in　material　design　for　practical　high-power　applications　such　as　pharmaceutical　extraction　and　ultrasonic　osteotomes.