An　extremely　long　ribbon-shaped　polyoxometalate　(POM)　octamer,　{Eu16Co7Se16W128O448-(CIT)(10)(HCIT)(2)(NO3)(4)(OH)(4)(H2O)(52)}　(H4CIT　=　citric　acid),　with　a　maximum　length　of　ca.　6　nm　has　been　obtained,　which　is　the　longest　molecular　POM　reported　to　date　and　the　first　Dawson-based　3d-4f-containing　selenotungstate(IV).　The　nanoscale　octamer　is　built　from　8　new　Dawson-type　Eu2W2-substituted　selenotungstates　{(Eu2W2O4)Se2W14O52}　bridged　by　12　H4CIT　ligands　and　3　Co2+　ions,　giving　rise　to　a　rare　giant　inorganic-organic　hybrid　molecular　POM　with　more　than　150　metal　centers　(7　Co-II,　16　Eu-III,　16　Se-IV,　and　128　W-VI).　Especially,　with　128　W　centers　and　more　than　800　non-H　atoms,　the　POM　octamer　also　represents　the　largest　known　3d-4f-containing　polyoxotungstate.