A series of Cu70Ni30 alloy ingots, each weighing 650 g, were solidified with various undercoolings prior to nucleation. The material was mechanically tested in the as-solidified condition. Although the decrease of dendrite segregation with increasing undercooling is favorable for the improvement of mechanical properties, the sophisticated evolution of structural morphology leads to a non-monotonic change of both elongation and strength. The maximum tensile strength is obtained in the quasi-spherical structure which forms above the critical undercooling 200 K, while the maximum elongation occurs in the longitudinal tension of the columnar dendrite structure solidified at undercooling 180 K.