A directional sensor network is composed of many directional sensor nodes. Unlike conventional omni-directional sensors that always have an omni-angle of sensing range, directional sensors may have a limited angle of sensing range due to technical constraints or cost considerations. Area coverage is still an essential issue in a directional sensor network. In this paper, we study the area coverage problem in directional sensor networks with mobile sensors, which can move to the coverage holes to get better coverage ratio. In order to increase the coverage ratio, each sensor will move from overlapped regions to coverage holes after random deployment. The movements of sensors are adjusted round-by-round so that the coverage ratio is gradually improved. To guide the moving direction of each sensor, we introduce a virtual forces based coverage algorithm for directional sensor networks. Our proposed scheme consists of four different forces caused by neighboring sensors and uncovered regions in the field. By applying the resultant force from these four different forces to directional sensors, coverage holes are eliminated quickly. Simulation results show the effectiveness of our scheme in term of the coverage improvement.