The formation of a plate shaped ω phase in Zr–Nb alloy after shock deformation has been studied with a view to ascertaining the nature of this transformation. The orientation relationship between the β and the ω lattices was {1 1 1}_β//(0 0 0 1)ω and (1 1‾ 0)_β//(1 1 2‾ 0)ω which is identical to that seen in case of ω phase forming in this alloy on thermal treatment. The experimentally determined habit plane of the plate shaped ω phase has been compared with that predicted from the phenomenological theory of martensite formation. A mechanism of transformation involving shear on the planes has been considered. The importance of mechanical instability of the b phase in bringing about this transformation has been investigated. The mechanism of initiation of ω transformation has been ascertained by high resolution electron microscopy (HREM) of the β lattice and compared with that happening during ω formation in the same alloy by thermal treatment.