A display panel includes a plurality of pixel areas and at least one inspection area. An incident light is irradiated onto an inspection pattern disposed in the inspection area and a reflection light reflected by the inspection pattern is detected. An optical critical dimension of the inspection pattern is calculated from the reflection light, and a dimension of a pixel pattern disposed in each pixel area is calculated from the optical critical dimension of the inspection pattern. Accordingly, the dimension of the pixel pattern may be indirectly measured from the inspection pattern.