The improvement of the mechanical properties such as tensile strength, hardness, and ductility were studied by a new and developed technique in the additions field (ZnO nanorods additions) under as-cast condition. Different melting temperatures, holding times, and ZnO nanorods amount were used and the refining of the microstructures and comparison of the obtained results with heat treatment conditions were also investigated in this paper. The results showed that the network of eutectic silicon particles is agglomerated, refined, and less angular after adding of ZnO nanorods to A356 aluminum casting alloys with quick solidification by water. Good enhancement in ultimate tensile strength and elongation with slightly increasing in the hardness were observed and they reached 222 MPa, 35%, and 78 HB respectively. Moreover, ZnO nanorods additions to A356 aluminum casting alloys give the possibility to improve the ultimate tensile strength, elongation, and hardness without heat treatment. Furthermore, the optimal melting and holding temperature for the present work is 700°C with holding time of 3 hours.