A phase diagram of the alloys of the Al-Cu-Ga ternary system has been determined by using X-ray analysis.The isothermal section at room temperature was found to consist of eleven single-phase regions (i.e. α, γ2, γ′, δ, ζ1, ζ2, η2, θ, θ′,αA1 and Ga), fourteen two-phase regions (i. e.α + ζ1, α+ γ2, α + γ′, γ2 + γ′, γ′+ ζ1, γ2+δ, γ′+δ,δ +ζ2, ζ2+η2, η2+θ,η2+ θ′, γ′ + θ′, θ′+ Ga, and θ + αA1); and nine three-phase regions (i. e. α + γ′+ζ1, α+ γ2+ γ′, γ2+ γ′ + δ,γ′ + δ + θ′, δ+ζ2+θ′, ζ2 + η2 + θ′, η2+ θ′ + Ga, η2 +θ + Ga and θ + Ga + αA1). No new phase has been observed in this system.