We report on a new technology to detect the surface defect (SD) density of AlGaAs related materials by wet thermal oxidation. After oxidation, the color around the defect differs from that of the surrounding under an optical microscope. The defect density can be easily counted even in the case of low density. Because the oxidation temperature, 370degreesC, is much lower than layer-growth temperature, oxidation does not induce additional SD. The density counted in this way is exactly the real one. (C) 2004 Elsevier B.V. All rights reserved.