Ferromagnetic shape memory alloys can produce large strains in a magnetic field by twin boundary motion. The mobility of parent-martensite inter-phase interfaces and twin-twin inter-variant boundaries is closely related to their interfacial structures and the mechanism of migration, therefore a thorough understanding of its nature is of importance. A physical model of the structure of parent-martensite interface has been developed recently based on dislocation theories and topological arguments. On the basis of the topological model, the present work performs a theoretical study of the defect structure of the martensitic interface, and the transformation crystallography in Ni2MnGa alloy. The habit plane, i.e., the parent-martensite interface plane was determined to be (-0.761, -0.054, -0.646)P for the parent crystal index frame, and (-0.818, 0.067, -0.571)M for the martensite frame. The habit plane inclination angle is 5.945° and 5.953° with respect to the terrace plane in the parent and martensite crystals, respectively.