In this paper, Fe-Fe50 wt.%Si diffusion couples are subjected to 1200℃ heat treatment in static magnetic field. Fe-Fe50 wt.% Si diffusion couples were prepared by vacuum casting technology and later sectioned and polished for scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies before heat treated. Microstructures of the treated samples which were polished first were analyzed by SEM and EDS; results show that the phase components of the interfacial intermetallic compound layers are FeSi phase layer and Fe-Si solid solution layer whether the samples were treated with or without magnetic field, and the layer widths in the samples treated with magnetic field are smaller than those without magnetic field. According to the parabolic law, the interdiffusion coefficients of the interfacial intermetallic compound layers were calculated and the interdiffusion coefficients of FeSi phase and Fe-Si solution under a field of 0.8 T are reduced by 26.7% and 34.1%. The Gibbs energy due to applied field was calculated, data analysis shows that the reduction of interfacial intermetallic compound layer coefficients is attributed to the decrease of frequency factor, not the activation energy. Decrease in layer thickness decrease is suggested to be related to the retardation of atomic diffusion resulting from the magnetic field, and a possible theory based on Larmor precession is given to explain this effect.