The Be- and Mg-content-dependent phase transition of BeMg ZnO alloy is investigated by theoretically calculations and experiments. For a given content of Be, the maximum content of Mg at which the wurtzite structure still remains in BeMgZn O alloy is intensively studied. We find that as the content of Be increases, the maximum content of Mg in wurtzite Be MgZn O alloy increases accordingly. Moreover, the Be- and Mg-content-dependent band gap of the alloy at each wurtzite-rocksalt transition point is evaluated, which is expressed as an empirical law. In addition, the mechanism underlying the tunable band gap of the alloy in wurtzite Be MgZn O alloy is revealed. © 2014 Elsevier B.V. All rights reserved.