Trisilanol phenyl polyhedral silsesquioxane (phenyl TSP) was added to a commercial aluminum alloy, AlSi10MnMg, to investigate its influence on microstructure, mechanical properties, and solidification behavior. Addition of phenyl TSP was successful in refining the morphology of the eutectic Si into an ultrafine lamellar structure. Furthermore, this refined as-cast morphology can be obtained even when the molten aluminum alloy was held for 96h in a furnace set at around 1000K. This no fading phenomenon is of a significant processing benefit to the current use of Sr for refining the eutectic morphology. The ductility of phenyl TSP-refined AlSi10MnMg alloy was consistent with Sr-modified alloy at room, 423K, and 573K elevated temperature while maintaining similar strength. Cooling curve analysis showed the Al-Si eutectic arrest temperatures during solidification were decreased with the phenyl TSP addition, suggesting phenyl TSP bonds with Al to slow down the Al segregation from Al-Si melt during the eutectic reaction, leading to the microstructural refinement of Al-Si eutectic.