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Well-separated pair decomposition, introduced by Callahan and Kosaraju [3], has found numerous applications in solving proximity problems for points in the Euclidean space. A pair of point sets (A, B) is c-well-separated if the distance between A,B is at least c times the diameters of both A and B. A well-separated pair decomposition of a point set consists of a set of well-separated pairs that “cover” all the pairs of distinct points, i.e. any two distinct points belong to the different sets of some pair. In [3], Callahan and Kosaraju showed that for any point set in an Euclidean space and for any constant c ≥ 1, there always exists a c-well-separated pair decomposition with linearly many pairs. This fact has been very useful in obtaining nearly linear time algorithms for many problems such as computing k-nearest neighbors, N -body potential fields, geometric spanners, approximate minimum spanning trees etc. Well-separated pair decomposition is also shown very useful in obtaining efficient dynamic, parallel, and external memory algorithms. The definition of well-separated pair decomposition can be naturally extended to any metric space. However, a general metric space may not admit a well-separated pair decomposition with a sub-quadratic size. Indeed, even for the metric induced by a star tree with unit weight on each edge1, any well-separated pair decomposition requires quadratically many pairs. This makes the well-separated pair decomposition useless for such a metric. However, it has been shown that for unit-disk graph metric, there do exist well-separated pair decompositions with almost linear size, and therefore many proximity problems under unit disk graph metric can be solved efficiently.