A powder sample of Sr 3 FeMoO 7 was synthesized by solid-state reaction in reduced atmosphere (5% H 2 /Ar). At room temperature, Sr 3 FeMoO 7 crystallizes in a typical Ruddlesden–Popper (n=2) structure in the space group I4/mmm, a=3.9309(2)Å and c=20.435(2)Å. The structure refinement indicates that the Fe and Mo ions are randomly distributed in a single B-site with small fraction of B-site and oxygen vacancies. At low temperature, long-range magnetic interaction was observed. The antiferromagnetic magnetic interaction can be described with a large unit cell, am=bm=2an and cm=cn, in the magnetic space group An′.