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• Fully dense and grain-refined (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Eu 0.2) 2 Zr 2 O 7 high-entropy nanocrystalline ceramics were fabricated at 800 °C under 10 GPa. • The grain size of UHPS-ed sample was only 151 nm, which is 40% smaller than that of original powder. • Ultra-high pressure contributed to the unique microstructure with refined grain, high density of dislocations and sub-grains. • UHPS-ed sample showed higher hardness and better aqueous durability than the conventional pressureless sintered one. As an important A 2 B 2 O 7 -type ceramic, (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Eu 0.2) 2 Zr 2 O 7 high-entropy pyrochlore possesses promising properties such as high melting point, high chemical durability, and low thermal conductivity. However, the low sintering ability limits its application in thermal barrier coating and radioactive waste immobilization. It usually needs long-term high-temperature soaking to achieve full density, but with inevitable grain growth. In this work, dense and grain-refined nanocrystalline (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Eu 0.2) 2 Zr 2 O 7 ceramics were prepared with ultra-high pressure sintering (UHPS) method under 10 GPa at a low temperature of 800 °C. The densification behavior, microstructure evolution, and properties of the UHPS-ed samples were then investigated. The grain size of as-prepared (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Eu 0.2) 2 Zr 2 O 7 ceramic was only 151 nm, which is 40% smaller than that of raw powder. In addition, it exhibited advantageous properties including both high hardness and aqueous durability. Plastic deformation under ultra-high pressure was believed as the dominant densification mechanism responsible for grain refinement and property improvement. [Display omitted] [ABSTRACT FROM AUTHOR]