A locating device for measuring distances between dots of a light guide plate includes a sheet having a plurality of reference points marked thereon. The plurality of reference points is configured for locating of the dots during measuring, and a distribution density of the reference points is lower than a distribution of the dots. A preferred method for using a locating device to measure distances between dots of a light guide plate includes the steps of: providing the locating device having a plurality of reference points marked thereon, the plurality of reference points being configured for locating of the dots, a distribution density of the reference points being lower than a distribution density of the dots; attaching the locating device to a surface of the light guide plate that is opposite to the dots; and gauging distances between the dots using a microscope.