Using　average　30　nm　grain　sized　potassium　sodium　niobate　powders　obtained　by　a　novel　Sol-Gel　method　as　starting　materials,　nanocrystalline　potassium　sodium　niobate　ceramics　with　pure　orthorhombic　phase,　relative　density　of　above　99%　and　grain　size　of　40　nm　were　prepared　by　spark　plasma　sintering　method　under　sintering　temperature　of　900　degrees　C,　sintering　pressure　of　30　MPa　and　sintering　time　of　1　min.　The　phase　structure,　micro-morphology,　dielectric　properties　and　ferroelectric　properties　of　the　ceramics　were　investigated.　The　results　show　that,　differently　from　common　micro-grain　ceramics,　dielectric　constant　of　nanocrystalline　potassium　sodium　niobate　ceramics　decreases　to　341,　which　changes　little　with　the　temperature,　and　the　ceramics　have　obvious　dielectric　relaxation　(dispersion　factor　gamma　is　1.60),　and　a　well　saturated　ferroelectric　hysteresis　loop　with　coercive　field　of　13.5　kV/cm　and　remanent　polarization　of　1.5　mu　C/cm(2).　The　abnormal　properties　can　be　attributed　to　the　significant　increasing　content　of　grain　boundary　in　nanocrystalline　potassium　sodium　niobate　ceramics,　and　it　can　be　expected　that　if　potassium　sodium　niobate　ceramic　has　the　critical　size　at　room　temperature,　it　would　be　less　than　40　nm.