— A directional sensor network consists of a number of directional sensors, which can switch to several directions to extend their sensing ability to cover the interested targets in a given area. Because a directional sensor has a smaller angle of sensing range or even does not cover any target when it is deployed, how to cover the interested targets becomes a major problem in directional sensor networks. In this paper, we address the directional cover set problem (DCS) of finding a cover set in a directional sensor network, in which the directions cover all the targets. We propose both centralized and distributed algorithms for the DCS. We also introduce two applications that utilize these algorithms to extend the network work time while maximizing the coverage of the targets. Simulation results are presented to demonstrate the performance of these algorithms and the applications.