The effect of yttrium addition on fundamental characters of magnesium has been investigated by calculated and experimental methods. It was found that the lattice parameter increased and the axial ratio c/. a decreased with the increment of yttrium in solid solution. The approximately linear relationship between Young's modulus and the content of yttrium in solid solution was observed in the single phase solid solution Mg-Y alloys (T4 state). However, Young's modulus was closely relevant to the fraction of second precipitate except the content of saturated yttrium in matrix for the aged Mg-Y alloys (T6 state). The concentration of vacancy increased with the increased content of yttrium in solid solution, which was mainly caused by the atomic size misfit and the difference of valence electrons between yttrium and magnesium.