Microhardness, aluminium content and phase constitution characteristics in the Fe-Al alloy layer of calorized steel pipe were investigated by optical microscopy, microhardness measurements, SEM, electron probe microanalysis, TEM and X-ray diffractometry, etc. Experimental results indicate that the Fe-Al alloy layer of calorized steel pipe was mainly composed of FeAl phase (20%-36% Al), Fe3Al phase (13.9%-20% Al) and alpha-Fe (Al) solid solution, and the microhardness in the Fe-Al alloy layer was 600-310 HM from the surface layer to the inside. There were no higher aluminium content phases, such as brittle FeAl2, Fe2Al5 and FeAl3. The ability to resist deformation and the weldability of the calorized steel pipe were remarkably improved.