A double sandwich polyoxometalate and its Fe(III) substituted derivative, [As2Fe5Mo21O82](17-) (1) and [As2Fe6Mo20O80(H2O)(2)](16-) (2), were synthesized and characterized by single-crystal X-ray diffraction, infrared spectroscopy, fluorescent spectroscopy, UV spectra, thermogravimetry-differential scanning calorimetry analyses, electrospray ionization mass spectrometry, and magnetism measurements. The polyoxoanion is composed of a central fragment FeMo7O28 for 1 (Fe2Mo6O26(H2O)(2) for 2) and two external AsMo7O27 fragments linked together by two distinct edge-sharing dirneric clusters Fe2O10 to lead to a C-2v, molecular symmetry. The central FeMo7O28 fragment and external AsMo7O27 fragment have a similar structure, and both of them can be viewed as a monocapped hexavacant alpha-Keggin subunit with a central FeO4 group or a central AsO3 group. Both of the polyoxoanions contain a oxo-bridged Fe-5(III) magnetic core with the angles of Fe-O-Fe in the range of 96.4(4)-125.7(5)degrees, and magnetism measurements show an overall ferromagnetic interactions among the five-nuclearity cluster Fe-5 with the spin ground state S = 15/2.