The alkali and alkali-earth metals doping were widely investigated to improve the electroluminescence characteristics of organic light-emitting diodes (OLED). We study the effects of magnesium (Mg) incorporation in the transport layer, and also the emission layer of the OLED. A thermally deposited Mg has been inserted into the various positions, and the thickness was varied either. By the position and the thickness, the Mg made partially an electronic channel and made a leakage path ways. Resulted difference of initial maximum luminance distribution was observed. On the other hand, with the optimized inserting position and thickness of Mg, the OLED showed the improved EL characteristics.