This paper aims to unveil effects of alpha colony and alpha plate parameters on tensile properties and impact toughness of Ti-55531 alloy with multilevel lamellar microstructures. Microstructures with different alpha colony and alpha 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 alpha colony (d(c)) and alpha plate (d(alpha)) and increasing of volume fraction of alpha phase. Both d(c) and d(alpha) follow the Hall-Petch principle with strength, and d(c) was determined to be the critical controlling parameter of strength owing to higher interfacial strengthening effect induced by alpha colony. However, plasticity is primarily related to d(alpha). Moreover, impact toughness for both d(c) and d(alpha) follows the linear relation, but the coefficient of d(alpha) is higher. Furthermore, impact toughness is more dependent on plasticity than strength. Larger alpha 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 alpha plates. [GRAPHICS] .