A method for inspecting a critical dimension may include providing a substrate, applying a photoresist on the substrate, variably irradiating a dose of light onto the photoresist, performing a photo process to develop the photoresist to form a photoresist pattern, performing an etching process using the photoresist pattern as an etching mask to form a plurality of patterns, measuring a width of each of the plurality of patterns and a spacing between adjacent ones of the plurality of patterns, and identifying a cause of a defect in the photo process based on the measured width and the measured spacing.