The characteristics and the mechanism of ferrite transformation in alloy steels which contain a carbide-forming element have attracted considerable attention for past decades. Since it is reported that the nucleation and growth of ferrite in Fe-C base alloys is accelerated by high magnetic field, the influence of a magnetic field of 12 Tesla on ferrite transformation was studied in a Fe-C- Mo alloy. Whereas a significant amount of expedition was observed at lower temperatures, the principal features of ferrite transformation, namely, a marked retardation of transformation at intermediate temperatures and premature cessation of transformation before it reaches the final equilibrium amount below the bay temperature were essentially retained. In contrast, the influence of magnetic field was much less at higher temperatures. These results are discussed in terms of the influence of magnetic field on the phase equilibrium and coupled-solute drag effects on the migration a/g phase boundary.