A new mathematical approach of the phenomenological theory, similar to the Kajiwara model, has been applied to the martensitic transformation of BCC (B2) to monoclinic 7R close-packed structure, in the Ni-37.0 at% Al alloy. Analytical equations for the habit plane indices and the magnitude of the lattice invariant shear were derived. The results obtained from the present theory were compared with those of phenomenological crystallographic theories and experimental results. (c) 2005 Elsevier B.V. All rights reserved.