Defect-modulated　Al-Ni-Rh　crystalline　approximants,　which　are　structurally　related　to　a　decagonal　quasicrystal　with　1.6　nm　periodicity,　have　been　found　to　form　in　an　as-　solidified　Al75Ni15Rh10　alloy.　Their　structural　features　were　studied　by　a　combination　of　high-　resolution　electron　microscopy　(HREM)　and　high-angle　annular　dark　field　scanning　transmission　electron　microscopy　(HAADF-STEM).　The　results　show　that　the　prefect　structures　of　Al-Ni-Rh　crystalline　approximants　are　characterized　by　periodic　arrangements　of　hexogen　units　(H)　constructed　from　atom　columnar　clusters.　Defect-modulated　structures　were　formed　due　to　ordered　arrangement　of　one-　or　two-dimensional　structural　defects　of　high　density.　The　basic　structural　defect　is　associated　with　incorrect　arrangements　of　atom　columnar　clusters　which　appear　as　polygons　different　from　the　H　units.　These　polygons　of　atom　columnar　clusters　represent　a　type　of　linear　defect　extending　along　the　columnar　axis　direction,　and　they　can　array　to　form　a　planar　defect.　By　means　of　the　HAADF-STEM　imaging　technique,　the　structural　nature　of　the　core　area　for　the　linear　defect,　which　usually　exhibits　dark　contrast　in　HREM　observations,　has　been　clearly　revealed.