The precipitation behavior in Mg-6 wt%Zn-1 wt%Y alloy annealed at different temperatures of 200˚C and 400˚C has been characterized by high resolution transmission electron microscope. When the alloy is annealed at 200˚C for 6 hr, the plate- and the rod-shaped β2' phases are precipitated in the matrix. The orientation relationship of plate-shaped precipitates with the matrix exhibits a [11ar20]β2 || [10ar10]Mg, (0001)β2' || (0001)Mg. While the rod-shaped precipitates have two kinds of the orientation relationships with the matrix, i.e. [11ar20]β2'||[0001] Mg, (0001)β2'||(11ar20) Mg and [11ar20]β2'||[0001] Mg, (ar1106)β2'||(10ar10) Mg. With increasing annealing time at 200˚C the β1' phases are also precipitated in the matrix and the orientation relationship exhibits a [010]β1' || [0001]Mg, (603)β1' || (01ar10)Mg between the β1' precipitate and the matrix. The icosahedral phases are precipitated in the alloy annealed at 400˚C and exhibit a [I2]I || [0001]Mg relationship with the matrix.