InGaN/GaN vertical light emitting diodes (LEDs) with argon (Ar) and oxygen (O2) plasma-treated nonalloyed Al/Ti electrodes were fabricated on sapphire substrates. At the operating current of 350 mA, the forward voltage (VF) for O2 plasma-treated Al/Ti-based devices with dimensions 1360 × 1360 μm2 was improved, whose value was comparable or lower to that of nonalloyed Cr/Au-based devices. The Al/Ti electrodes resulted in improvement in optical output power of LEDs due to their high reflectivity (typically 10%–15% higher based on our data) compared to LEDs with conventional Cr/Au-based electrodes. The x-ray photoelectron spectroscopy showed the increase in Ga-O peak intensity during O2 plasma treatment. These results demonstrate that O2 plasma-treated Al/Ti electrodes reduced the contact resistance by forming a thin conductive GaOxN1-x layer at n-GaN surface.