The constituent of the whole ternary system copper-aluminium-silicon were considered. No ternary intermetallic compound is observed and 15 definite constituents which have. been determined in each binary system were also found to occur in this ternary system. There exist 11 surfaces of the primary separation, and 8 invariant points on the liquidus surface; among them 2 points are ternary eutectic, 1 point ternary peritectic, and 5 points peritecto-eutectic. Both the β phases of the Cu-Si and Cu-Al system are completely soluble in all proportions in the ternary system, and it decomposes into α+γt; the univariant reaction line of this decomposition has a minimum point at about 500°C. 2 invariant reactions in solid state, γ_t+γ_s χ+α, γt+χ α+ε_s", are found to exlst at 675° and 495°C. Five invariant reactions; γt+ε_2a δ+Si, ε_2a δ+ζ₁+Si, ζ₁ δ+ζ₂+Si, ε_2a+η₁ ζ₁+Si, ζ₁+η₁ ζ₂+Si and two ternary polymorphic changes due to the η₁ η₂ phases, take place almost the same temperatures as in the binary Cu-Al system. Four ternary polymorphic changes due to the ε_s ε_s' and ε_s' ε_s" take place at 600° γt+ε_s S_s'+X, 550°γt+ε_s' ε_s"+X, 585°γt+ε_s ε_s'+Si, and 525°γt+ε_s' ε_s"+Si respectively. The limit of solubility of silicon and aluminium in Solid copper has been also determined.