Undoped Mgx Zn1-x O thin films with Mg content of 0≤x≤0.20 were grown on c -sapphire substrate by plasma-assisted molecular beam epitaxy. The Mgx Zn1-x O shows n -type conduction in Mg content of x≤0.05, and the carrier concentration decreases slowly from 1018 to 1017 cm-3 with increasing Mg content. However, as x0.10, the Mgx Zn1-x O begins to show p -type conduction, and the carrier concentration goes down sharply to 1015 cm-3 firstly and then increases slowly with increasing Mg content from 1015 to 1016 cm-3. The mechanism of transformation from n to p type and change of the carrier concentrations with Mg content were investigated by photoluminescence and absorption measurements as well as first-principle calculation. © 2007 American Institute of Physics.