A binary Al–Co decagonal quasicrystal has formed after 100 keVCo+-ion implantation into pure Al with a dose of1.5×1017 ion/cm2,and the quasicrystal has shown a high thermal stability. The composition of stable decagonal quasicrystal is believed to beAl11Co4. Co+ion implantation into Al first produced a multiple layer surface structure. The first layer was an amorphous layer. The second layer consisted of a decagonal quasicrystal and a coexisting amorphous phase. The amorphous phase in the implanted region where composition is close toAl11Co4 was transformed into the decagonal quasicrystal during annealing in a temperature range between 550 and 600 °C. Possible transformation mechanisms for the experimental results are discussed. ©1997 American Institute of Physics.