By using a high-temperature deformation method, two kinds of defect have been introduced into the Al70Co15Ni15decagonal phase. They are firstly high–density stacking faults with fault planes perpendicular to the tenfold axis, and displacement vectors R lying in the fault planes and parallel to a twofold axis A2D and secondly high–density dislocations with Burgers vectors parallel to the tenfold axis. Some dislocations exist as dislocation pairs. On the basis of experimental observations we have made the first estimate of the antiphase–boundary energy of the ordered Al70Co15Ni15decagonal phase (about 30erg cm–2) and explored the microscopic mechanism of high-temperature deformation of the decagonal quasicrystal.