Abstract-In this paper we investigate performance of a ZigBee based wireless sensor network in two scenarios one with a static sink and other with random sink mobility through extensive ns2-based simulations for various topologies (star, tree and mesh). Wireless Sensor Networks (WSNs) can be considered as a glut of distributed sensor nodes which are able to sense a physical process or environmental parameters and also capable to transfer that information to a predefined sink node through some intermediate nodes. If sink remains static at a fixed position then neighboring nodes of the sink consume their energy more rapidly as compared to other nodes which are far away from the sink because they have to forward all the traffic of the farther away nodes to the sink that leads to complete isolation of sink in a less time and hence network performance is degraded due to non-uniform consumption of energy. We introduce a simple random mobility scheme in which sink moves randomly through whole network. We also investigate end-to-end delay of packets and throughput for identical network conditions in both the cases. The main aim of this work is to evaluate, through simulations, the impact of random sink mobility in a ZigBee/IEEE 802.15.4 based wireless sensor network.Index Terms-Random mobility, WSN, ZigBee, IEEE 802.15.4, sink.