An optical encoder includes a light emitting module, a positioning device and a light separating structure. The light emitting module emits a light beam illuminating an illumination area of the positioning device. The positioning device includes light penetrating areas arranged in a dislocation manner. The light penetrating areas sequentially move into the illumination area. The light separating structure is disposed in the path of the light beam. The positioning device is disposed between the light emitting module and the light separating structure. When part of the light beam penetrates one of these light penetrating areas and is transmitted to the light separating structure, the light separating structure transmits the light to a sensing area and forms at least one first positioning optical pattern. The distance between two first positioning optical patterns formed by two adjacent light penetrating areas is greater than the pitch between the two adjacent light penetrating areas.