The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface.