Two kinds of GaN/AlN superlattices (SLs) with different thickness strained layers have been grown on Si(l 1 1) substrate using pulsed laser deposition (PLD) assisted by direct current discharge. The optical and electrical properties of the GaN/AlN SLs were characterized with X-ray diffraction (XRD), Van der Pauw-Hall measurements, micro-Raman spectroscopy, and photoluminescence (PL) spectroscopy. It is observed that the background carrier concentration and electron mobility in the GaN/AlN SL with thicker strained layers is bigger than that with thinner strained layers. From the Raman spectra, the experimental data of thinner strained layers GaN/AlN SL are found to be in agreement with the results of previous reports. For the GaN/AlN SL with the thicker strained layers, the GaN E-2 Peak is absent and the other peaks are obviously broad and weak. In the PL spectra, the SL with thinner strained layers only has strong band edge emission peak. But the SL with thicker strained layers has weak yellow luminescence peak beside weaker band edge emission peak. These suggest that the thickness of strained layers in the GaN/AlN SLs have significant effect on the optical and electrical properties. (C) 2003 Elsevier Science B.V All rights reserved.