In order to measure steel corrosion in mortar by a transient electro-magnetic (TEM) Method, the development of the sensors have been studied. The sensors were made of enamelled wire with diameter of 0.25mm and Acril. The sensor configuration was used as a coincident loop type. The secondary electro motive force(EMF) was measured with SIROTEM III. The accelerator was equipped with the SIROTEM III. The accelerator permits the transmitter to turn off approximately 10~15 times faster than normal. The high resolution time series used for very shallow or high resistivity investigation was selected. The steels were embedded in mortar which were made from sand : cement : water ratio of 2 : 1: 0.5. The mortar specimen was 50cm long, 20cm wide and 10cm thick. To investigate steel corrosion in mortar, the sensors used were with 2$ imes$2$cm^2$(3, 6, 9$Omega$), 3$ imes$3$cm^2$(3, 6, 9$Omega$) and 6$ imes$6$cm^2$(3, 6, 9$Omega$). The obtained result obtained showed that the sensor 8(6$ imes$6$cm^2$, 6$Omega$) was the proper sensor for the measurement of steel corrosion in mortar.