We have studied the effect of p-GaN:Mg cap layers which were grown using MOCVD with various flow rates of Cp2Mg on the turn-on voltage of p-n junction LED. Before fabricating the LEDs, the p-GaN:Mg/sapphire (0 0 0 1) epilayers grown with various flow rates of Cp2Mg were evaluated in order to investigate the characteritics of each layer. We investigated the dependence of acceptor concentration on the flow rate of dopant source. The Van der Pauw technique, double crystal X-ray diffractometry (DCXRD:) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN/sapphire (0 0 0 1) epitaxy increases, the resistivity of the epilayers increases abruptly without discontinuity due to the increase of the much uncracked Mg-H complex. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers increases at first and then decreases from a certain amount of Mg incorporation. As a result of the increase of the hole concentration, the emission intensity of the layers in the :PL spectra at room temperature increases at first and then decreases from a certain flow rate of Cp2Mg. After full evaluation of each p-GaN:Mg epitaxial layer, LED structures of the p-n junction type were grown by the growth of these cap layers and fabricated. Next, they were characterized by I-V measurement in order to investigate the effect of p-GaN : Mg cap layers on the turn-on voltage of the LED structures. The current of the LEDs at forward bias mode becomes low as the resistivity of p-GaN : Mg cap layer increases. However, the LED fabricated by the cap layer which has the highest hole concentration shows the greatest current value. While the turn-on voltage of pn junction LED fabricated by p-GaN:Mg having high hole concentration above 1 x 10(17)/cm(3) depends on the hole concentration of each p-GaN:Mg cap layer, the voltage of the devices fabricated by the cap layer having low hole concentration below 1 x 10(17)/cm(3) is nearly dependent on the electrical resistivities of each p-GaN:Mg.