In this study particle redistribution and grain size refinement induced by hydrostatic extrusion (HE) have been studied in two Al-Si cast alloys (Al-9%Si and Al-11%Si). It has been found that HE results in a significant changes in particle shape, size and distribution which was revealed by SEM observations and quantified using stereological parameters. At the same time, significant grain refinement down to ~100 nm in diameter takes place in aluminium phase. Such a microstructure evolution affects substantially the mechanical properties of two-phase alloys. The yield strength and tensile strength increase over two times whereas the plasticity only slightly decreases.