The p-type Zn_0.8Mg_0.2O/n-type ZnO heterojunctions have been fabricated by a pulsed laser deposition (PLD) technique and the relations between the I-V and the electroluminescence (EL) characteristics were investigated. The ZnMgO film shows the best p-type characteristic for the 10% Sb concentration. The different Ga concentrations in n-type ZnO layers show different I-V and EL characteristics for the ZnMgO/ZnO heterojunctions. The 0.1% of Ga in the ZnO film shows the lowest resistivity (0.048 ohm·cm) and the highest electron mobility (59 cm²/Vs), however, the ZnMgO/ZnO junction shows only an Ohmic contact and no EL is observed. The heterostructure using the ZnO layer with 5.0% Ga shows the best rectifying behavior, however, no EL is observed. The 1.0% of Ga in the ZnO film shows the middle resistivity (0.51 ohm·cm) and the middle electron mobility (27 cm²/Vs) and the ZnMgO/ZnO heterojunction shows only a weak rectifying behavior but clear EL is observed. The threshold voltage of the EL is about 10V and the EL intensity is increased with the forward biases up to 20 V. These results indicate that the EL from ZnMgO/ZnO junctions does not necessarily require good ideality factors of the diodes. [DOI: 10.1380/ejssnt.2009.366]