In a method and an apparatus for inspecting defects on a substrate using a light beam, a light source irradiates light beams having different wavelengths onto the substrate. A detector detects first lights scattered from a surface of the substrate and second lights scattered from impurities on the substrate by irradiation of the light beams. An operation unit compares first intensities of the first lights with second intensities of the second lights in order to produce differential values therebetween, and selects a wavelength corresponding to a maximum value of the differential values. An inspection process for inspecting the defects on the substrate is performed using a light beam having the selected wavelength.