Ceramic-based　dielectric　materials　are　regarded　as　the　best　candidates　for　advanced　pulsed　power　capacitors　because　of　their　excellent　mechanical　and　thermal　properties.　Nevertheless,　lead-free　bulk　ceramics　show　relatively　low　recoverable　energy　storage　density　(W-rec　＜　2　J　cm(-3))　owing　to　their　low　dielectric　breakdown　strength　(DBS　＜　200　kV　cm(-1)).　In　order　to　significantly　increase　W-rec,　we　proposed　a　strategy　(compositions　drive　the　grain　size　to　submicrometer)　to　improve　the　DBS　of　lead-free　ceramics.　In　this　work,　(1-x)(K0.5Na0.5)　NbO3-xSrTiO(3)　(KNN-ST)　ceramics　were　chosen　as　a　representative　to　verify　the　validity　of　this　strategy.　The　(1-x)　KNN-xST　ceramics　(x-0.15　and　0.20)　with　submicrometer　grains　(about　0.3　mu　m)　were　prepared　using　pressureless　solid　state　sintering.　A　large　W-rec　(4.03　J　cm(-3))　and　DBS　(400　kV　cm(-1)　with　a　thickness　of　0.2　mm)　were　achieved　for　0.85KNN-0.15ST　ceramics.　The　value　of　4.03　J　cm(-3)　is　superior　to　all　other　W-rec　in　lead-free　bulk　ceramics　and　2-3　times　larger　than　that　of　other　lead-free　bulk　ceramics.　A　large　W-rec　(3.67　J　cm(-3))　and　energy　storage　efficiency　(72.1%)　were　simultaneously　achieved　for　0.80KNN-0.20ST　ceramics.　The　results　confirm　that　the　(1-x)　KNN-xST　ceramics　(x-0.15　and　0.20)　are　desirable　materials　for　advanced　pulsed　power　capacitors.　The　findings　in　this　study　could　push　the　development　of　a　series　of　KNN-based　ceramics　with　enhanced　DBS　and　W-rec　in　the　future.　On　the　other　hand,　this　work　could　broaden　the　applications　of　KNN　materials　in　a　new　field.