ZigBee is a recent wireless standard based on IEEE 802.15.4 for Personal Area Networks. Its use in Wireless Sensor Networks arouses many interests. In this paper, a performance analysis and an improvement of ZigBee routing protocol are carried out. ZigBee routing protocol uses a modified AODV by default and Hierarchical Tree Routing as last resort. Firstly, these two algorithms are compared in terms of delay performance and energy consumption. The results showed that Hierarchical Tree Routing provides shorter average end to end delay but performs poorly in terms of energy consumption. So for supporting real time communication, it is desirable to freely choose one or another according to the type of traffic (real-time and non real-time). Secondly, Hierarchical Tree Routing algorithm is slightly modified to provide shorter delays than the original one.