The effect of size refinement of primary silicon and modification of eutectic silicon, separately and simultaneously, on microstructure of hypereutectic Al-20wt%Si has been investigated under different casting conditions. Results showed that number per unit volume of primary silicon particles was doubled by inoculation with 0.02% phosphorus in the alloy with 0.0015% phosphorus already in it, whereas 70% of a small ingot (40×35 mm) became modified eutectic, and the formation of primary silicon was suppressed, on adding 0.1% strontium. Simultaneous addition of phosphorus and strontium still refined the primary silicon but did not modify the eutectic. Addition of strontium alone caused roughening of the primary silicon, exemplified by increased undercooling registered on the cooling curve. Also crystallographic relationships between polyhedral primary silicon particles have been investigated by Electron Back Scattered Diffraction (EBSD).