Lead-free piezoelectric ceramics (Na1-xKx)(Nb1-ySby)O-3+z mol% MnO2 have been prepared by a conventional solid-state sintering technique. Our results reveal that Sb5+ diffuses into the K0.5Na0.5NbO3 lattices to form a solid solution with a single-phase orthorhombic perovskite structure. The partial substitution of Sb5+ for B-site ion Nb5+ decreases the paraelectric cubic-ferroelectric tetragonal phase transition (T-c) and the ferroelectric tetragonal-ferroelectric orthorhombic phase transition (TO-F), and retains strong ferroelectricity. A small amount of MnO2 is enough to improve the densification of the ceramics. The co-effects of MnO2 doping and Sb substitution lead to significant improvements in ferroelectric and piezoelectric properties. The ceramics with x=0.45-0.525, y=0.06-0.08, and z=0.5-1 exhibit excellent ferroelectric and piezoelectric properties: d(33)=163-204 pC/N, k(P)=0.47-0.51, k(t)=0.46-0.52, epsilon=640-1053, tan delta=1.3-3.0%, P-r=18.1-22.6 mu C/cm(2), E-c=0.72-0.98 kV/mm, and T-C=269 degrees-314 degrees C.