Crystalline　approximants　structurally　related　to　decagonal　quasicrystal　in　the　as-cast　and　heat-treated　Al75Pd15Fe10alloys　and　defect　structures　in　them　have　been　studied　by　means　of　high-resolution　electron　microscopy　(HREM).　Structural　defects　of　linear　and　planar　types　were　found　to　exist　extensively　in　the　orthorhombic　Ε16-phase　formed　in　the　as-cast　Al75Pd15Fe10alloy.　In　contrast　with　the　distribution　and　configuration　of　the　defects　in　the　as-cast　Ε16-phase,　we　found　that　high-temperature　heat　treatment　promotes　the　formation　of　a　kind　of　regular　network　of　structural　defects　in　the　Ε16-phase.　This　suggests　that　rearrangements　of　atom　clusters　and　as　well　as　defects　occurred　due　to　the　heat　treatment.　The　relationship　between　the　distribution　of　atom　clusters　and　the　configuration　of　defects　will　be　discussed.