Physical parameters pf wirtzote Zn 1-xMg xO (0 ≤ x ≤ 1) were analyzed using first-principles calculations. A planewave pseudopotential method was employed to a density functional theory, then the effect of Mg incorporation on its lattice parameters and spontaneous polarization in the c-axis direction was studied as a function of the Mg content x. Calculations showed only a little difference in the bond length while a considerable change in the bond angle by the replacement of the Zn atoms in Zn 1-xMg xO with Mg atoms. Due to the atomic replacement, a linear decreases and increase with increasing x were brought anout the c-axis and a-axis lengths, respectively, deducing an increase of the spontaneous polarization n the -c-axis direction. Cohesive energies of wurtzite and rocksalt Zn 1-xMg xO were aksi calculated to compare the structural stability, and the former structure was shown to be more stable itll the Mg content at around x = 0.5.