Aiming at the problem that the existing GPRS monitoring system has no signal in some areas and the expensive price, an on-line monitoring system for transmission line based on 6LoWPAN technology is proposed. 6LoWPAN technology has built a bridge for communication between the wireless sensor and the Internet for the system, and achieved data collection, analysis to the transmission line temperature, wind speed and other monitoring variables and the alarm about fault prediction. In this paper, the 6LoWPAN technology is introduced, the gateway nodes and sensor nodes are analyzed from the way of the hardware and software structure, the gateway is composed of CC2530 and S3C2410 processor, CC2430 and ATmega128L are used to constitute the sensor node; it also analyzes the advantages of 6LoWPAN energy saving and designs the power supply. From the test results can be seen that the system has the advantages of practicability, high efficiency, saving cost, etc on the detection of transmission line.