Abstract: For long freezing range A1-4.5%Cu alloy plate castings, the effect of mischmetal and lanthanum on dendrite arm spacing, mechanical properties and macrosegregation is investigated under sand casting in this paper. The relationship between solidification parameters and dendrite arm spacing and ultimate tensile strength of castings was studied. The results of experiment show that addition of RE and La to AI-4.5%Cu alloy decreases its dendrite arm spacing. By addition of 0.2%RE, ultimate tensile strength and ductility of casting increased byl4%oand24%respectively. By addition of 0.5% RE, the macrosegregation of whole plate castings well decrease. The relationship between solidified parameters, dendrite arm spacing and ultimate tensile strength was analysed by statistical analysis. It shows that freezing index and gradient acceleration parameter are of two efficient parameters which control dendrite arm spacing, and a linear relation ship exists between dendrite arm spacing and ultimate tensile strength of castings. It provide basis for controlling quality of casting and evaluating ultimate tensile strength of.castings with nondestructive inspection method.