Polycrystalline powders of the family of compounds Sr2Fe1-xAl xMoO6 (x=0, 005, 010, 015, 030) were prepared according to the standard solid_state reaction . XRD patterns and SEM images show that the structure and the grain size of these compounds were not changed and no imp urity was observed by doping with Al in Sr2FeMoO6. In the cationic ordered s egregation, a large ferrimagnetic patch can be separated into some smaller ones by the Al doping. So the chain of the ferrimagnetic exchange path will be short ened and the strength of exchange will be weakened. Therefore, their moments bec ome more sensitive to external field. Furthermore, when the Al ion replaces the Fe ion in the disordered segregation, the antiferromagnetic coupling becomes wea ker. As a result, the low_field magnetoresistance is enhanced. But the smaller f errimagnetic segregates become more sensitive to thermal agitation at high temp eratures. The weakness of the Al_doped SFMO lies in its stronger temperature dep endence of the saturated moments and the low_field magnetoresistance.