The structure and growth morphology of Al₆₅Cu₁₅Fe_20−xCo_x and Al₆₅Cu₂₀Fe_15−xCo_x alloys have been examined in rapidly solidified, conventionally solidified and fully annealed states. A conventionally solidified Al₆₅Cu₁₅Co₂₀ alloy, which was prepared by the whole substitution of Fe by Co for a stable icosahedral Al₆₅Cu₂₀Fe₁₅ quasicrystal, was found to consist of a stable decagonal quasicrystal with a columnar morphology. The length of the decagonal column is as large as about 0.15 mm and the growth is thought to take place along a preferential direction parallel to the tenfold periodic axis because the plane perpendicular to the longitudinal direction of the column has the morphology of five dendrite arms radiating from the dendrite centers. The finding of the stable decagonal quasicrystal is important in the clarification of the formation mechanism and fundamental properties of the quasicrystals with an icosahedral or a decagonal structure.