Based on the basic principle of wireless sensor networks, the communication platform of a new ZigBee wireless ad hoc network is applied to the on-line monitoring system of transmission lines. In this article, the hardware and software platform architecture of the system is introduced, the scheme and algorithm for cluster network of monitoring nodes based on ZigBee technology are described; moreover a reliable, energy-saving and efficient data transmission method is designed. Application results show that ZigBee technology has the features such as self-organization, dynamic topology, large network capacity and data-centeredness, and possess a distinct advantage in on-line monitoring of electrical equipments.