1. An iris extraction method comprising the steps of: (A) defining two searching regions in a face image, wherein each searching region is a rough position of an eye for locating an iris; (B) measuring an energy of each pixel within the searching region according to a deformable template match (DTM) algorithm and an energy function, wherein each pixel is a center of a plurality of hypothetical circular templates, each having a hypothetical radius between a maximal radius and a minimal radius, for measuring the energies of the hypothetical circular templates in the face image; (C) recording the pixels with the same hypothetical radius having energies greater than a threshold as iris candidates, and recording a pixel having the maximal energy from the iris candidates as a first iris candidate; (D) comparing two of the iris candidates in each searching region for detecting a plurality-of associated iris pairs, and recording the lower iris candidate having the maximal energy of the iris pairs as a second iris candidate; (E) selecting the pixel corresponding to the second iris candidate as a best iris candidate if the distance between the first iris candidate and the second iris candidate is smaller than a predetermined radius; and (F) designating the best iris candidate having the maximal energy of all best iris candidates with different hypothetical radius as an iris in the face image.