A method for measuring a distance D2 between two points includes following steps. A first surface of a portable electronic device is parallel to a line defined by the two points. A distance D2 between the first surface and the line is obtained. A visible light beam B1 is rotated from an initial direction substantially perpendicular with the first surface and the line to direct at the point E1. A first angle defined by the visible light beam B1 striking the point E1 and the initial direction is computed. A visible light beam B2 is rotated from an initial direction to strike the point E2. A second angle defined by the visible light beam B2 striking the point E2 and the initial direction is computed. A distance D1 is computed based on the distance D2, the first angle and the second angle. The distance D1 is outputted.