We have observed the quantum Hall-effect (QHE) of ZnO/Mg_xZn_1−xO bilayers grown on ScAlMgO₄ substrates by using pulsed-laser deposition. Two-dimensional electron gas was spontaneously formed in the interface due to the polarization mismatch between the layers. In order to obtain high electron mobility, we first developed a high-temperature annealed buffer layer, and then carefully investigated the growth temperature dependences of surface morphologies and electrical properties. As the result, electron mobility was recorded to be 440 cm²/Vs at room temperature and 5500 cm²/Vs at 1 K. The observation of QHE allows to access direct determination of the electronic structure of the heterointerface.