In this paper, we introduce a new type of coverage for wireless sensor networks, called Directed Coverage (DCoverage). Basically, D-Coverage is the coverage provided by a sensor network monitoring an area between two boundaries, through which the intruder attempts to penetrates the area. We also study how to measure the quality of D-Coverage. Our first evaluation approach is a projection-based simple approach, while our second approach is a more comprehensive Markov chain based approach. Our evaluation approaches can accurately evaluate the quality and provide good guidelines for sensor network deployment and run-time repair.