The characteristics of phase constitution near the interface of Fe3Al/Q235 diffusion bonding are researched by means of scanning electron microscope, X-ray diffraction and transmission electron microscope, etc. The test results indicated that an obvious diffusion transition zone forms near the Fe3Al/Q235 interface as a result of vacuum diffusion bonding (heating temperature 1050-1080℃, holding time 60min and pressure 9.8MPa). The maximum value of the Al content 13 in the transition zone was 16.6wt.% (about 28.5 at.%). The micro-hardness in the diffusion transition zone was HM 14 200-400. The transition zone consists of Fe3Al and a-Fe(Al) solid solution. There was no brittle phase of high hardness 15 near the diffusion interface. This is favorable to the enhancing of the toughness of Fe3Al/Q235 diffusion joint.