We give an algorithm that computes the pathwidth of a given directed graph D in 3τ(D)nO(1) time where n is the number of vertices of D and τ(D) is the number of vertices of a minimum vertex cover of the underlying undirected graph of D. This result extends that of [5] for undirected graphs to directed graphs. Moreover, our algorithm is based on a standard dynamic programming with a simple tree-pruning trick, which is extremely simple and easy to implement. An additional advantage of our algorithm is that a minimum vertex cover appears in the analysis but not in the algorithm itself, in contrast to the algorithm of [5] which needs to compute a minimum vertex cover.