The way in which martensitic transformation in iron alloys in high pulsed magnetic field can proceed via two different routes is explained. The first route corresponds to the Clausius–Clapeyron equation and can be considered “typical structural phase transition”. The second route is governed by the redistribution of the exchange-interaction forces between electrons in the fcc-lattice phase under the action of external factors which increase the interatomic distance (magnetostriction of paraprocess). Such a transformation route can be regarded as “the first-order magnetic phase transition”. In magnetic field, in some alloys the “structural phase transition” changes to “the first-order magnetic phase transition”. The hypothesis that the martensitic transformation in some steels and iron alloys can be regarded as “the first-order magnetic phase transition” even in the absence of a magnetic field (expansion of the lattice by spontaneous magnetostriction, carbon doping, etc.) is expressed.