1. A method of measuring a critical dimension of a pattern, the method comprising: obtaining a pattern image from an object pattern; matching a design pattern of the object pattern and the pattern image to determine a detection region on the pattern image; determining a plurality of optimum turning points of pattern contours in the detection region, wherein a set of points at predetermined intervals along each of the pattern contours in the detection region are determined, and a single point from the set of points for each of the pattern contours is detected as an optimum turning point based on a characteristic of a tangent line; setting a ROI (region of interest) within a predetermined range from each of the determined optimum turning points; and measuring a critical dimension of the pattern in the ROI.