High magnetic fields were applied to the austenite to proeutectoid transformation and tempering process in a 42CrMo steel. The thermodynamic and kinetic effects of the high magnetic field on the austenite decomposition show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases. Moreover, the magnetic field can considerably lower the amount of low angle misorientations of ferrite in pearlite colonies and obviously increase the frequency of S3-29 coincidence boundaries, especially S3 boundaries, of the ferrite. But it has no obvious effect on crystallographic orientation distribution. When the field is applied to the high temperature tempering process, it can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. The magnetic field also obviously retards the formation and growth of the ‘distortion-free’ regions of the matrix.