The Young's modulus for a series of binary Mg–Gd and Mg–Nd alloys are studied in the present work. Fine and homogeneous grain structures are prepared by using hot extrusion. The results demonstrate that the Young's modulus of Mg–Gd alloys increase linearly by the increase of Gd in solid solution. Aging treatments are applied to the Mg–0.79–2.43 at% Gd alloys. A needle-like orthorhombic structure β′ phase is formed in Mg matrix. Due to a higher Young's modulus of the intermetallic β′ phase which is estimated to be 80 GPa, the Young's modulus of Mg–Gd alloys are enhanced by aging. The results for Mg–Nd alloys indicate that Young's modulus firstly decreases and reaches 42.53 GPa for Mg–0.18 at% Nd which is attributed to the solid solution of Nd in Mg. The Mg₄₁Nd₅ particles appear in Mg matrix when Nd content is higher than 0.18 at%, and Young's modulus of the particles is tested as 57.0 GPa. Thus, the Young's modulus increases to 43.42 GPa for Mg–0.63 at% Nd. The Young's modulus of Mg alloys are affected by altering the crystal cell parameters with solid solutes, and/or the formation of precipitate phases with varying amounts.