A device and method for measuring a critical dimension of a pattern on a display substrate is disclosed. In one aspect, the device includes a region of interest (ROI) setting unit setting a region of interest in image data, determining whether the region of interest is larger than a reference region, and generating a pattern image based on the region of interest. The device also includes a design file memory storing a plurality of design patterns, a matching unit matching the pattern image to one of design patterns, and a measurement unit measuring the critical dimension of the pattern in the pattern image. The ROI setting unit selects the image data as the pattern image and outputs the pattern image to the matching unit when the region of interest is larger than the reference region.