The standard aluminum alloy AlSi9Cu3(Fe) is the most common Al alloy used to produce castings by means of high-pressure casting technology and has wide application in the automotive industry, especially for motor mounts, gearbox housings, filter housings, generator housings, turbocharger housings, etc. The advantages of AlSi9Cu3(Fe) alloy are relatively good mechanical properties, strength and hardness, although it shows slightly lower elasticity and corrosion resistance. The artificial aging process or stabilization annealing can be used to improve the dimensional stability of castings, with the aim of achieving better mechanical properties as soon as possible. In this paper, the influence of natural aging time on the mechanical properties of AlSi9Cu3(Fe) alloy as well as on microstructure development was investigated. Specimens were prepared by high pressure die casting technology under optimized parameters. Chemical content was determined by optical emission spectroscopy. Thermodynamic calculation was performed by Thermo-Calc 5 software and transformation temperatures during dynamic measurement were followed by differential scanning calorimetry. Tensile properties after 3 and 30 days of aging, as well as 7 months, were determined by a tensile test at an MTS 810 instrument. Metallographic analysis was carried out by optical microscope and field-emission scanning electron microscope. Results showed a significant increase in tensile strength, yield strength and elongation with aging time and formation of (α Al + β Si ), Al x (Fe,Mn) y Si z , Al 2 Cu-Si, Al 5 FeSi and Al-Mg 2 Si-Cu phases.