One of the most important problems in the ophthalmic image processing is to precisely localize an optic disk in a retinal image. Due to the fact that existing OD localization methods which commonly utilize the OD features such as size, brightness, shape, contrast and location often fail to localize OD on the low quality retina images in which such features are lacked, a new method namely Vessel Vector Based Phase Portrait Analysis (VVPPA) is proposed. The newly proposed OD localization method exploits Phase Portrait Analysis (PPA) technique to find the convergence of the vectors derived from retina vessels. The detail of the technique, the quantitative results, and the numerical results will be presented.