Metal–matrix composites (MMCs), as light and strong materials, are very attractive for application in different industries. In the present work, nano and micro-composites (A356/Al2O3) with different weight percent of particles were fabricated by two melt techniques such as stir-casting and compo-casting. Microstructural characterization was investigated by optical (OP) and scanning electron microscopy (SEM). Tensile, hardness and compression tests were carried out in order to identify mechanical properties of the composites. The results of microstructural study revealed uniform distribution, grain refinement and low porosity in micro and nano-composite specimens. The mechanical results showed that the addition of alumina (micro and nano) led to the improvement in yield strength, ultimate tensile strength, compression strength and hardness. It was indicated that type of fabrication process and particle size were the effective factors influencing on the mechanical properties. Decreasing alumina particle size and using compo-casting process obtained the best mechanical properties.Highlights► Nano and micro-composites (A356/Al2O3) were fabricated by stir-casting and compo-casting. ► Uniform distribution, grain refinement and low porosity in the composites were attained. ► Addition of alumina led to the improvement in yield, ultimate tensile and compression strength. ► Nano-alumina particles and compo-casting process obtained the best mechanical properties.