Due to formability and simplicity of production process, excellent mechanical properties and combination with other metals, aluminum alloys are widely used in all industries. Alloying elements such as silicon, magnesium or copper and trace elements such as iron, manganese, boron, titanium, strontium or sodium are added to improve the mechanical, physical and corrosion properties of aluminum alloys. Their impact will contribute to the targeted development of microstructures that will ultimately improve the mechanical properties. An important role plays the choice of casting technology, casting parameters that will affect the cooling and / or solidification rates, and therefore the development of the microstructure and the final application value of the castings. The theoretical part describes the properties and use of aluminum and its alloys, the influence of chemical composition and cooling rate on mechanical properties and microstructure. The solidification process of Al alloys is also clarified. In the experimental part of the work, an experiment was conducted in the foundry Lipovica d.o.o. For the experiment, we used an AlSi12 alloy cast by a high-pressure casting process. The aim of the study was to evaluate the influence of wall thickness on the microstructure and mechanical properties of AlSi12 alloy. Chemical composition analysis, microstructure analysis, static tensile test and hardness measurement were performed on samples with different wall thicknesses. The research established that the wall thickness has an impact on the microstructure and mechanical properties of AlSi12 alloy.