An extensive phase transition of (Na1−xLix)NbO3 (x=0.04, 0.1) from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by mechanical stress. A reverse phase transition was induced by thermal annealing at temperatures as low as 300°C. A sintered pellet was subjected to mechanical stress by grinding to powder form in a mortar. The orthorhombic phase in the ceramic pellet transformed to R3c phase, by as much as 83–89% by grinding. The net cell volume of the R3c increased by 0.4–0.5Å3 per four formula unit (z=4). The reverse transition from the R3c to Pmc21 was 84–97mass% after annealing at 300°C. The activation free energy for the reverse transition was calculated to be 2–10meV using an Arrhenius-type equation. The crystal class of the Pmc21, i.e. mm2, conformed to the measured P–E hysteresis (Ps≅20μC/cm2) of the pellet. •An extensive phase transition in Na1−xLixNbO3 was induced by mechanical stress.•Orthorhombic phase in the sintered pellet transformed to a rhombohedral phase.•The extent of the transformation was ≥83mass% by grinding in a mortar.•A reverse phase transition occurred by annealing at temperatures as low as 300°C.•The activation energy for the reverse phase transition was 2–10meV.