The algorithm for finding the supporting line of two planar ploygons is a kernal algorithm in many computation geometry problems and has found wider applications in computer graphics and robotics.So far
the computation complexity of the best algorithm for this problem is O(n+m). In this paper
with the help of the properties of the convex ploygons
a fast algorithm with time complexity O(log (n+m ) )is presented and is proved to be optimal in the range of constant factors.