Abstract： The co-existence of two transition phases β′1 and β′2 at the initial stage of 165℃ aging of a Mg-5% Zn alloy was shown by means of X-ray diffraction method. The one-dimensional diffraction phenomena of the thin rod-like transition phase β′2 were investigated in detail. This transition phase has a hexagonal lattice structure, its long axis is the 6-fold symmetry axis (β′2). The lattice constants of β′2 and its orientational relationship with the magnesium matrix are found as follows:a(β′2)=13.2?,c(β′2)=5.25?,(β′2)∥M,<1010>(β′2)∥<1010>M. The problem concerning the lattice distortion of the matrix by precipitation of the β′2 phase has been discussed.