The effects of a high magnetic field on phase transformation behaviors and microstructures in Fe-based alloys have been extensively studied. It was found that a magnetic field accelerates ferritic and martensitic transformation, changes the morphology of the transformed microstructures and increases the A3 and A1 temperature. In a magnetic field of 10 Tesla, the A1 temperature increases by about 15°C for Fe-0.8C, the A3 temperature for pure Fe increases by about 8°C and the martensitic transformation temperature Ms in 18Ni maraging steel increases by 20°C. Ferrite grains are elongated and aligned along the direction of magnetic field in Fe-0.4C and Fe-0.6C alloys by ferritic transformation, but elongation was not found in pure Fe, Fe-0.05C alloy and Fe-1.5Mn-0.11C-0.1V alloy. Aligned structure was not found either by pearlitic transformation in Fe-0.8C alloy or by cementite precipitation from martensite.