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 A(3) and A(1) temperature. In a magnetic field of 10 Tesla, the A(1) temperature increases by about 15 degrees C for Fe-0.8C, the A(3) temperature for pure Fe increases by about 8 degrees C and the martensitic transformation temperature M-s in 18Ni maraging steel increases by 20 degrees 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.