Composition dependence of crystallographic features of α″-martensite in Ti–Nb-based alloy was systematically examined in Ti–Xmol%Nb–3mol%Al alloy, where X is 10–30. One of the lattice deformation strains, η3, became 0 at the critical Nb concentration of 23 mol% and the sign of η3 changed across the composition. The phenomenological theory of martensite crystallography (PTMC) predicted that the lattice invariant shear (LIS) would vanish at the critical composition. Transmission electron microscopy revealed that non-twinned martensite was formed with a habit plane of {755}b when η3 = 0, and that martensite with an internal twin of {111}o type I was formed with a {443}b habit plane for η3 = +0.008. Subscript b and o indicate β and α″ lattice, respectively. The crystallography of these two α″-martensites were well explained by the PTMC. On the other hand, non-twinned martensite with {111}b–{775}b habit plane was formed even when η3 was not zero. It was found that there is a critical value of |η3| for the formation of twinned martensite and that class B transformation, in which LIS is a {011}o compound twin, hardly occurs in β-titanium alloys.