The phase diagram of the ternary Gd-Co-Cu system with Gd content 34.8 wt% at room temperature has been determined by X-ray diffraction.The isothermal section was found to consist of five singlephase regions (i. e. α, σ,δ,γ and θ),seven two-phase regions (i. e. α+β, β+δ,δ+θ,θ+γ,γ+α,θ+α and σ+θ) and three three-phase regions (i.e.α+β+θ,α+θ+γ and β+θ+δ). No new phase has been observed. X-ray analysis showed that maximum solubility of Cu in Co is about 4wt% and maximum solubility of Gd in β-phase is about 3wt%.