The modification effect of Sr addition on the microstructure of Al-Si casting alloys (AlSi9Cu1 and AlSi9Cu3Fe) was investigated. The modification of eutectic in Al-Si casting alloys is a generally accepted process used primarily to improve mechanical properties by promoting the refinement of the inherently coarse and plate-like eutectic silicon phase. Furthermore, the combined effect of Sr addition and cooling rate was also studied in this paper because the microstructure of the eutectic is significantly affected by the cooling rate. The following measurements were used in this research: optical microscopy, measurement of secondary dendrite arm spacing, computer image analysis, scanning electron microscopy and electron microprobe analysis. Keywords: Al-Si alloys, strontium, eutectic modification, cooling rate.