A rectilinear path between two points p, q ∈ R 2 is a path connecting p and q with all its line segments horizontal or vertical segments. Furthermore, a Manhattan path between p and q is a rectilinear path with its length exactly dist(p, q) := |p.x − q.x| + |p.y − q.y|.Given a set T of n points in R 2 , a network G is said to be a Manhattan network on T , if for all p, q ∈ T there exists a Manhattan path between p and q with all its line segments in G. For the given point set T , the Minimum Manhattan Network (MMN) Problem is to find a Manhattan network G on T with the minimum network length.In this paper, we shall prove that the decision version of MMN is strongly N P -complete, using the reduction from the well-known 3-SAT problem, which requires a number of gadgets. The gadgets have similar structures, but play different roles in simulating the 3-SAT formula. The reduction has been implemented with a computer program.