A method of fabricating a semiconductor device is disclosed. The method may include forming a parent pattern, forming an upper thin film on the parent pattern, forming a child pattern on the upper thin film, measuring a diffraction light from the parent and child patterns to obtain an intensity difference curve of the diffraction light versus its wavelength, and performing an overlay measurement process on the parent and child patterns using the diffraction light, which has the same wavelength as a peak of the intensity difference curve located near a peak of reflectance of the parent and child patterns, to obtain an overlay measurement value.