The fracture behaviour of DO3Fe3Al-based intermetallic alloys with and without Mn (1.5 at %) were investigated by tensile tests (TT), transmission electron microscope (TEM) and scanning electron microscope (SEM). The results show that the addition of Mn could improve mechanical properties of the alloy, including room temperature ductility and high temperature strength. DO3Fe-28Al fractured in a transgranular cleavage mode at room temperature and it gradually changed to a ductile style with temperature increasing, whereas the sample with addition of Mn fractured mainly in a mixed intergranular-transgranular cleavage mode. Three major factors are considered to have effect of Mn on fracture behaviour of the alloys : reducing grain size, promoting slip and cross slip and enhancing cleavage strength.