A locating method applied to an optical touch device is disclosed. The optical touch device includes at least one image detecting component for detecting an imaging position of an object and generating a touch position of the object on a plane according to the imaging position. The at least one image detecting component includes a plurality of pixels. The locating method includes dividing the pixels into a first group and a second group along a first direction, analyzing dimension ratios of a darkness point respectively relative to the first group and the second group when the darkness point formed by the object is simultaneously detected by the first group and the second group, and calculating the imaging position of the darkness point inside the image detecting component according to the dimension ratios.