Microstructures and phase compositions of as-cast and solution-treated ADC12 and ADC12-0.85Y alloys were studied by optical microscope, scanning electron microscopy and X-ray diffraction method. Meanwhile, the impact toughness was tested. The results showed that as-cast microstructure was refined significantly when 0.85%Y was added into ADC12 alloy. The coarse primary α-Al dendrite changed into fine cellular and equixed grains, and the Si-rich phase changed from long needle or plate to short rod or globosity. Meanwhile, few acicular new phase Al3Y precipitated. When the two alloys were solution-treated at 793 K for 8 h, the phase compositions did not change and the partial second phases dissolved into α-Al matrix. However, the long needle or plate Si-rich phase disappeared completely, and the remaining second phases precipitated by short rod or globosity. Meanwhile, the segregation degree between dendrites decreased. When 0.85%Y was added, the impact toughness value increased from 3.8 J/cm2 for as-cast ADC12 alloy to 10.8 J/cm2, and further reached 21.9 J/cm2 after the solution treatment, with the amplitude of 476% compared with the corresponding as-cast value of ADC12 alloy. The impact fracture exhibited the typical ductile fracture with dimples.