The present study details the effects of hydrogen peroxide in the cerium nitrate solution on the microstructure and growth of the conversion coating on AZ31 magnesium alloys. Adding hydrogen peroxide to the cerium nitrate solution changed its color from clear to orange; meanwhile Cr(superscript 3+) was oxidized to Ce(superscript 4+). The major overlay of the coating was cerium oxide that had peeled partially from the substrate due to hydrogen evolution at the interface. Increasing solution temperature from 30 to 60℃ resulted in more serious peeling, when the coating, as characterized by cross-sectional TEM, exhibited a multilayer structure. That is, a porous metal a couple of cerium oxide layers loosely stacking on the surface of the coating.