Temperature has a direct effect on the operation of electrical devices. To meet the demands of the smart grid, which needs to apply the control, sensing and communication technology into the power system , it is indispensable to design the on-line monitoring system for temperature measurement. In this system, digital temperature sensors are employed to obtain the temperature value of the under-test devices in real-time and then the temperature data are sent to the monitor center through the cable transmission or the wireless transmission. The alerting signal will be sent to the monitor center at the same time. The function of monitor center is data processing and long-range controlling. To ensure the accuracy of the telecommunication, the on-line monitoring system adopts the communication protocol which is based on MODBUS. Via the realization of the system, line staff can know about the operation condition of the electrical devices without on-site measurements.