Spectroscopic studies have been conducted for wurtzite ZnO/Mg0.27Zn0.73O multiple quantum wells. Internal electric fields at interfaces are found to have an influence on the photoluminescence (PL) properties for well width (L-w) greater than 4.2 mn. These experimentally observed features are characteristic of the quantum-confined Stark effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the L-w, respectively. Results of the time-resolved PL study are also presented.