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The structural and mechanical response of pure body-centered cubic (BCC) Nb under shock loading were systematically studied. Shock-induced orthorhombic (α") martensitic transformations were observed in single crystals Nb with different orientations, in which dense lenticular α" martensite variants distribute uniformly along different directions by forming a hierarchical structure. Neighbouring martensite variants share a {111}< 110> α" twinning relationship between each other, and high-density dislocations are found distributing along the α"/BCC (β) interface. The formation of α" martensite was dominated by a mechanism of collinear shuffling of alternating {110} β planes along with< 1 1¯ 0> β direction by a displacement δ. The collision between martensite variants along different direction induce abundant microcracks, with BCC-to-ω and BCC-to-FCC phase transformations occurring at the crack tips. These …