The effect of a magnetic field on the formation of pearlite in a Fe-0.12%C steel was investigated. The results show that pearlite colonies elongate and align along the field direction, and that this tendency increases with increasing magnetic field strength. The possible preferential nucleation of ferrite between existing ferrite grains aligned along field direction at the later stage of proeutectoid transformation promotes carbon diffusion into the austenite areas between the ferrite chains accounts for the phenomena. Moreover, the field effect is dependent on the specimen position with respect to the field direction.