A reliable and accurate method to measure the width of retinal blood vessel in fundus photography is proposed in this paper. Our approach is based on a graph-theoretic algorithm. The two boundaries of the same blood vessel are segmented simultaneously by converting the two-boundary segmentation problem into a two-slice, threedimension surface segmentation problem, which is further converted into the problem of computing a minimum closed set in a nodeweighted graph. An initial segmentation is generated from a vessel probability image. Two datasets from the REVIEW database were used to assess the performance of proposed method. This algorithm is robust and is able to produce accurate measurements