Abstract: Fingerprint periodicity/frequency is an important parameter for filtering，segmentation，ridge line tracking and matching in Automated Fingerprint Identification System（AFIS）.Due to the influence of image definition，even a half-pixel error may cause periodicity/frequency calculation error，resulting in a subversive result in the post-processing.With the sub-pixel calculation，the fingerprint periodicity matrix and normal direction image can be gotten accurately and the sharp corner of fingerprint image center line in existing direction image method can be avoided，the AFIS performance also can be improved significantly.