The capacity of ad hoc wireless networks is constrained by the interference between concurrent transmissions from neighboring nodes. Gupta and Kumar have shown that the capacity of an ad hoc network does not scale well with the increasing number of nodes in the system when using omnidirectional antennas [6]. We investigate the capacity of ad hoc wireless networks using directional antennas. In this work, we consider arbitrary networks and random networks where nodes are assumed to be static. In arbitrary networks, due to the reduction of the interference area, the capacity gain is proven to be 2 when using directional transmission and omni reception. Because of the reduced probability of two neighbors pointing to each other, the capacity gain is 2 when omni transmission and directional reception are used. Although these two expressions look similar, the proof technique is different. By taking advantage of the above two approaches, the capacity gain is 2 when both transmission and r...