Lead-free MnO-doped 0.955K(0.5)Na(0.5)NbO(3)-0.045Bi(0.5)Na(0.5)ZrO(3) (Abbreviated as KNN-0.045BNZ) ceramics have been prepared by the conventional solid-state sintering method in reducing atmosphere (po2=1x10(-10)atm) and air. For ceramics sintered in reducing atmosphere, only Mn2+ ions exist in ceramics who preferentially occupy the cation vacancies in A-site at x=0.2-0.4, whereas Mn2+ ions substitute for Zr4+ ions in B-site to form defects (MnZr) at x>0.4. For ceramics sintered in air, mixed Mn2+, Mn3+, and Mn4+ ions coexist here. The Mn2+ ions preferentially occupy the cation vacancies in A-site at x=0.2-0.4 and then Mn2+ ions substitute for Zr4+ ions in B-site at x>0.4. Meanwhile, the Mn3+ ions and Mn4+ ions substitute for Nb5+ ions in B-site to form defects at x=0.2-0.8. The , and dipolar defects show a positive dipolar defect contribution (DDC) to the whereas the dipolar defects show a negative DDC to the . The dipolar defects and can help improve the temperature stability of . The 0.4% MnO-doped KNN-0.045BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d(33)=300pC/N and 0.2% MnO-doped KNN-0.045BNZ ceramics sintered in air possess optimal piezoelectric constant d(33)=290pC/N.