We have investigated the p-type doping characteristics such as crystallinity, electrical resistivity and optical properties of Al0.15Ga0.85N:Mg epilayers grown by MOCVD with various Mg incorporations. While the mirror-like surface without any defects such as cracks and hillocks can be seen in the undoped Al0.15Ga0.85N epilayer which was grown without any intentional doping of Mg, many hexagonal pyramids and small islands are observed in the p-Al0.15Ga0.85N:Mg which was grown with Mg incorporation rate of 50 nmol/min. Mg-doped Al0.15Ga0.85N layers exhibit little increase in the electrical resistivity with Mg incorporation until 40 nmol/min and then shows the abrupt increase above 50 nmol/min. The cathodeluminescene spectra of every Mg-doped layer show that the donor-to-acceptor (D-A) pair transition is dominated at 362 nm, which is red shifted by 19 nm against band edge luminescence emission of the undoped Al0.15Ga0.85N. layer. By calculating the relative intensity between D-A pair transition of Mg-doped Al0.15Ga0.85N and band edge emission of the undoped, the change of optical characteristics with different Mg doping level was evaluated. This result suggested that the residual Mg-H complexes in highly Mg-doped Al0.15Ga0.85N are not cracked at a certain activation process and are responsible for the abrupt decrease of optical luminescence characteristics. (C) 2003 Elsevier B.V. All rights reserved.