The crystal nucleation and growth mechanism during the formation of Ag _42.4 Cu _21.6 Sb₃₆ ternary eutectic are investigated under substantial undercooling conditions. The x-ray diffraction analysis shows that the solidified eutectic phases are not involved (Ag) within a wide undercooling range of 6–114 K. This indicates that under high undercooling condition, the phase constitution of Ag–Cu–Sb eutectic is different from that in the equilibrium phase diagram. With the increase of undercooling, the crystalline morphology of Ag _42.4 Cu _21.6 Sb₃₆ alloy transforms from the mixed structure of primary θ(Cu₂Sb), two pseudobinary eutectics and (ε+θ+Sb) ternary eutectic into a unique (ε+θ+Sb) ternary eutectic. The calculated results indicate that θ(Cu₂Sb) is the leading nucleating phase among the three eutectic phases. In addition, the growth morphology of primary ε(Ag₃Sb) compound in Ag₆₀Cu₆Sb₃₄ alloy exhibits the characteristics of solid solution and its orthorhombic dendrite grows along the ⟨111⟩ directions.