The synthesis of semiconductor ZnO-Mg and application for methylene blue photodegradation had been done. Semiconductor of ZnO was doped by Mg metal. The ZnO-Mg semiconductors were made with mass ratio (3.5:5), (2:3) and (2:1) and were characterized used XRD, UV-DRS (Diffuse Reflectance Spectroscopy) and SEM. The result of characterization using XRD showed a typical peak intensity at 2θ = 32.9Â…