Both the nuclear and magnetic structure of a complex perovskite with composition Sr3Fe2WO9 (SFWO) has been studied by neutron powder diffraction (NPD) at different temperatures. The most striking feature of this type of complex perovskite is the coexistence of ferrimagnetic and antiferroelectric types of ordering below T = 373 K. The NPD data at 10 and 300 K were refined with a model in the tetragonal space group 14/m. SFWO has a partially ordered perovskite structure, in which Fe and W cations distribute regularly to some degree at the B-sites in the ABO(3) perovskite lattice. For both temperatures magnetic diffraction peaks were registered, and a possible model for the magnetic structure was proposed in accordance with the ferrimagnetic properties of SFWO.