COOJA is a network simulator developed for wireless sensor networks. It can be used for high-level algorithm development as well as low-level device driver implementations for accurate simulation of wireless sensor networks before deployment. However, in a simulation Cooja assumes that the nodes are only equipped with omnidirectional antennas. There is currently no support for directional antennas. Due to the growing interest in the use of directional or smart antennas in wireless sensor networks, a model that can support directional antennas is essential for the realistic simulations of protocols relying on directional communication. This paper presents work on extending COOJA with a directional antenna model.