A method for measuring critical dimension (CD) includes steps of: scanning at least one area of interest of a die to obtain at least one scanned image; aligning the scanned image to at least one designed layout pattern to identify a plurality of borders within the scanned image; and averaging distances each measured from the border or the plurality of borders of a pattern associated with a specific type of CD corresponding to the designed layout pattern to obtain a value of CD of the die. The value of critical dimensions of dies can be obtained from the scanned image with lower resolution which is obtained by relatively higher scanning speed, so the above-mentioned method can obtain value of CD for every die within entire wafer to monitor the uniformity of the semiconductor manufacturing process within an acceptable inspection time.