Eu-doped GaN has been grown by gas source molecular beam epitaxy on two kinds of sapphire (0 0 0 1) substrates; one has regular atomic step (AS) surface and the other one has random roughness (RR) surface. The effect of the substrate on the structural and optical qualities of Eu doped GaN was investigated. The atomic force microscope showed the epilayer on the AS substrate owns better surface morphology at each growth step such as nitridation, buffer layer, undoped GaN and Eu doped GaN layer. In the asymmetric X-ray diffraction of (11 (2) over bar4), the full-width at half-maximum of the Eu.-doped GaN on the AS substrate was almost a half value compared with that on the RR substrate. The room temperature photoluminescence spectra showed red emission due to f-f transition of Eu3+ ion from both epilayers, however, the intensity of the red emission from the epilayer grown on the AS substrate is five times stronger than that on the RR substrate. The cause of the improvement in the structural and optical properties of Eu-doped GaN grown on the AS substrate is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.