An eye state detecting method, applied to an electronic apparatus with an image sensor, comprising: (a) defining a detecting range according to a possible location of a user's eye, wherein the detecting range is smaller than a maximum detecting range that the electronic apparatus can apply; (b) capturing a detecting image via the detecting range; and (c) determining whether the user's eye is in an opening state or in a closing state according a brightness of to the detecting image. The present invention also discloses a method applying a smaller determining range to determine if the user's eye is open or close.