The algorithm of iris location is a pivotal step in iris recognition and the speed and accuracy of location is the standard to evaluate the effectiveness of the algorithm. According to the grayscale characteristic of the pupil area, a fast iris location algorithm is presented based on image sub block. Firstly, find an approximate location area of the pupil roughly. Then, locate it accurately by using the methods of morphology and geometry. Finally, locate the outer circle with integral differential operator in the delimited area after pupil location. An experiment on iris location with 756 images in iris image library of version 1.0 of Chinese Academy of Sciences has been completed. The results with 98.80% accuracy and 1.5s average positioning time shows that the algorithm improves recognition speed notably.