The precipitation behavior of Mg-Y-Nd-Zr alloy was investigated by using transmission electron microscopy. The precipitation sequence of Mg-Y-Nd-Zr alloy was confirmed as follows; Supersaturated solid solution --> beta" --> beta' --> beta. The beta" phase was identified Mg3Nd having DO19 structure with the lattice constant of a = 2a(Mg) = 0.64 nm and c = c(Mg) = 0.52nm. The beta' precipitate was identified as Mg12NdY having base centered orthorhombic structure with the lattice constants a, b and c of 0.64, 2.23 and 0.52 nm, respectively. The beta phase was confirmed as equilibrium phase having the fee structure with the lattice constant of a = 2.22 nm.