Lead-free NaNbO 3 -BaTiO 3 piezoelectric ceramics were fabricated under a low oxygen partial pressure, which enables co-sintering with base metal Ni electrodes. The reducing atmosphere was accurately controlled by adopting a suitable mixture of Ar, H 2 , and CO 2 gases. By adding MnCO 3 , ZrO 2 , and Li 2 CO 3 , resultant NaNbO 3 -BaTiO 3 ceramics sintered in a reducing atmosphere have high resistivities, maintain high sintering densities, and exhibit piezoelectric constants equivalent or superior to samples prepared in air. The Curie temperatures of the prepared reductionresistant NaNbO 3 -BaTiO 3 samples depend on the amount of BaTiO 3 ; they can be maintained at approximately 200 °C or higher by setting the BaTiO 3 content to 10 mol%. Particularly, 5 mol% MnCO 3 , 2 mol% ZrO 2 , and 1 mol% Li 2 CO 3 -added (Na 0.9 Ba 0.1 )(Nb 0.9 Ti 0.1 )O 3 ceramics sintered under a low oxygen partial pressure (PO 2 = 10 -8 atm at 1250 °C) showed a piezoelectric constant (d 33 ) of 135 pC N −1 and an electromechanical coupling coefficient (k p ) of approximately 25%. © 2021 The Japan Society of Applied Physics 2. Experimental methods 2.1. Fabrication of NaNbO 3 -BaTiO 3 ceramics in air or a reducing atmosphere Samples were prepared by the solid-state reaction process. NaNbO 3 (Kojundo Chemical), BaTiO 3 (Sakai Chemical