A decaprismatic growth morphology has been found for the first time for the decagonal quasicrystal in the Al65Cu20Co15alloy slowly cooled from the melt. Substituting a little si for Al in this alloy, for example, Al62Si3Cu20Co15, results in quasicrystal decaprisms of 0·5 mm in diameter and several millimetres in length. These decaprisms showed possibly a 10/mmmmorphology which was proved later by electron and X-ray single crystal diffraction analyses. Moreover, the sharp selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) patterns show clearly a high degree of perfection and the X-ray precession photographs prove the single crystalline nature across a few millimetres of this decagonal quasicrystal.