We show that for any constant ε > 0, there is no Ω(log1−ε M)approximation algorithm for the directed congestion minimization problem on networks of size M unless NP ⊆ ZPTIME(npolylog n ). This bound is almost tight given the O(log M/ log log M)-approximation via randomized rounding due to Raghavan and Thompson. Categories and Subject Descriptors F.2 [Analysis of Algorithms and Problem Complexity]: Nonnumerical Algorithms and Problems General Terms Algorithms Keywords Hardness of approximation, directed graphs, congestion minimization