To study measurement of coated steel corrosion in mortar, a transient electro-magnetic(TEM) method was adapted. The sensors were made of enamelled wire with diameter of 0.25mm(transmitter), 0.1mm(receiver) and the secondary electro motive force(EMF) was measured with SIROTEMIII. The sensors configuration was used as in loop configuration. After coated steels were corroded by the salt spray during 3, 7, 15, 25days, they were embedded in mortar which were made from sand : cement : water ratio of 2 : 1 :0.5. To investigate coated steel corrosion in mortar, the sensors were used. ( sensorl - $T_x$ : $4{Omega}$, $R_x$ : $10{Omega}$, $3{times}3cm$, sensor2 - $T_x$ : $8{Omega}$, $R_x$ : $10{Omega}$, $3{times}3cm$, sensor3 - $T_x$ : $4{Omega}$, $R_x$ : $10{Omega}$, $6{times}6cm$, sensor4 - $T_x$ : $8{Omega}$, $R_x$ : $10{Omega}$, $6{times}6cm$). The obtained results showed that the secondary EMF was decreased with specimens of 3, 7days corroded coationg steel in mortar and then increased with specccimens of 15, 25days corroded one. And it was confirmed that measurement of coated steel corrosion in mortar by a transient electro-magnetic(TEM) method is possible.