Wireless sensor networks (WSN) consists of light-weight, lowpower and small size sensor nodes (SNs). They have ability to monitor, calculate and communicate wirelessly.In this paper we present a performance evaluation of ZigBee which is IEEE 802.15.4 standard, including the Physical (PHY) layer and Media Access Control (MAC) sub-layer, which allow a simple interaction between the sensors.We provide an accurate simulation model with respect to the specifications of IEEE 802.15.4 standard. We simulate and analyzed two different scenarios, where we examine the topological features and performance of the IEEE 802.15.4 standard using OPNET simulator. We compared the three possible topologies (Star, Mesh and Tree) to each other.