Application of multipath routing techniques in mobile ad hoc networks has been explored earlier, as multipath routing may help to reduce end-to-end delay, perform load balancing and consequently improve throughput. However, it has also been shown that the success of multipath routing in ad hoc wireless network depends on network topology and channel characteristics can severely limit the gain offered by multipath routing strategies. The most significant challenge to making the use of multipath routing protocols effective in this environment involves considering the effects of route coupling. Route coupling in wireless medium occurs when two routes are located physically close enough to interfere with each other during data communication. As a result, the nodes in multiple routes are constantly contending for access to the medium they share and can end up performing worse than a single path protocol. In this paper, we propose a notion of zone-disjoint routes in wireless medium where paths are said to be zone-disjoint when data communication over one path will not interfere with data communication in other path. The notion of zone-disjointness is used as route selection criteria. However, zone-disjointness alone is not sufficient for performance improvement. If the path-length (number of hops) were large, that would increase the end-to-end delay even in the context of zone-disjointness. So, it is imperative to select maximally zone-disjoint shortest paths. However, getting zone-disjoint or even partially zone-disjoint routes in ad hoc network with omni-directional antenna is difficult, since the transmission zone of each node is larger compared to that with directional antenna. Hence, one way to reduce this transmission zone of a node is to use directional antenna. In this paper, we investigate the effect of directional antenna on zone-disjoint multipath routing and evaluated its effectiveness in QualNet network simulator.