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This paper aims to unveil effects of α colony and α plate parameters on tensile properties and impact toughness of Ti-55531 alloy with multilevel lamellar microstructures. Microstructures with different α colony and α plate sizes were prepared using varied heat treatments and analyzed by X-ray diffraction, scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy. Relationships of microstructure and strength, plasticity and impact toughness were discussed. Results indicate that tensile strength increases, while plasticity and impact toughness of the alloy decline with decreasing effective sizes (width/length ratio) of α colony (dc) and α plate (dα) and increasing of volume fraction of α phase. Both dc and dα follow the Hall–Petch principle with strength, and dc was determined to be the critical controlling parameter of strength owing to higher interfacial strengthening effect induced by α colony. However, plasticity is primarily related to dα. Moreover, impact toughness for both dc and dα follows the linear relation, but the coefficient of dα is higher. Furthermore, impact toughness is more dependent on plasticity than strength. Larger α plates lead to more mechanical twins during impacting and result in higher plasticity and impact toughness. Therefore, the impact toughness should depend more on the size of α plates. In order to unveil the influence of α plate size on impact deformation and microcrack initiation behavior of Ti-55531 alloy, the microstructural deformation and crack nucleation characteristics in the impact crack nucleation region were detected by TEM in detail; then, the schematic illustration for microstructural deformation and crack nucleation characteristics was made according to experiment results. [ABSTRACT FROM AUTHOR]