Structural elongation and alignment in an Fe-0.4C alloy transformed in high magnetic fields has been studied by quantitative microscopy analysis. An elongated and aligned two-phase structure is formed in high magnetic fields by isothermal ferrite transformation both below and above Curie temperature. Equiaxed ferrite grains nucleate randomly at austenite grain boundaries and they become elongated by preferred growth along the direction of applied magnetic field. Below Curie temperature, the degree of elongation increases with increasing transformation temperature, whereas it decreases above Curie temperature. Small chemical driving force for ferrite precipitation and large magnetization of ferrite in high magnetic fields favor the formation of elongated structure.