4YSZ is generally used in fuel cells, oxygen sensors, hip and knee joint replacements and thermal barriers because of it excellent properties, including low density, high biocompatibility, good resistance against corrosion, hard phase and high melting point, and high ionic conductivity. However, 4YSZ with large grain has low resistance to abrasion and wear because of its low fracture toughness and low hardness at room temperature. The hardness and fracture toughness of 4YSZ can be improved by the addition of a second hard phase to form a nanostructured composite. Nanostructured materials have received considerable attention because they possess a high hardness and fracture toughness. Recently, nano-crystalline YSZ powder was fabricated by several methods. However, nanosized sintered YSZ has not been obtained due to fast grain growth during the conventional sintering method. In this study, nanostructured 4YSZ-BN composites with nearly full density were achieved using pulsed current activated sintering for 80 sec at a pressure of 80 Mpa. The addition of hexagonal BN to 4YSZ caused the nano scale structure to be retained in the ceramic by deterring grain growth. The crystalline size of the 4YSZ was reduced by the addition of BN. The addition of BN to 4YSZ greatly improved its mechanical properties (hardness and fracture toughness). (Received October 14, 2019; Accepted November 16, 2019)