ZigBee is a standard based on the IEEE 802.15.4 standard for wireless personal networks. Its use in Wireless Sensor Networks (WSNs) has aroused a great interest in the research community in the last years. In this paper, a synthetic study to analyze the performances and improve the routing protocols in WSNs is given and we were interested in routing mechanisms defined by the ZigBee standard. Multiple routing protocols have been developed to find optimized routes from a source to destination. ZigBee routing protocol uses a modified AODV by default and Hierarchical Tree Routing as last resort. The scope of this work is to improve the existing protocol to take the scalability into account and use it in tree topology in a variable network sizes up to hundreds of nodes. This paper describes the different routing protocols, our proposed solution ZBR-M and finally it presents and discusses the obtained results.