ZnO:Al is widely used as transparent conductive oxide (TCO) layer in several electronic and optical devices. To widen bandgap energy (E-g) of ZnO:Al, Mg was introduced to form Zn1 - xMgxO:Al. In this work, the Zn1 -xMgxO:Al films with different [Mg] / ([Mg] + [Zn]) ratios (Mg content (x)) on soda-lime glasses and polycrystalline Cu(In, Ga) Se-2 thin films were prepared by radio frequency magnetron co-sputtering of ZnO:Al and MgO targets. The power density applied to ZnO:Al target was constant at 144.3 W/cm(2), whereas that applied to MgO target is varied from 0 to 183.7 W/cm(2) to change the [Mg] / ([Mg] + [Zn]) ratio from 0 to 0.205. It is demonstrated that Eg of the Zn1 - xMgxO:Al is increased with the [Mg] / ([Mg] + [Zn]) from 0 to 0.205, while the basic crystal structure is ZnO. The Zn1 - xMgxO:Al films with small [Mg] / ([Mg] + [Zn]) ratios of approximately 0.064-0.139 possess the better film quality and higher Hall mobility than those of ZnO:Al since their crystalline diameters are enhanced with the decreased deep-defect levels. With the small [Mg] / ([Mg] + [Zn]) up to 0.139, the carrier concentration is decreased, which is beneficial to lower the free carrier absorption, while the resistivity is kept low. Consequently, the Zn1 - xMgxO:Al films with the small [Mg] / ([Mg] + [Zn]) ratio (about 0.064-0.139) are promisingly utilized as TCO layer.