Despite growing concern about electromagnetic radiation, the biological effects of the frequency magnetic fields remain obscure. The cortical neurons isolated from the mice were exposed to a 50 Hz electromagnetic field (EMF 1 mT, 5 mT, 10 mT) for 15 min. Using the whole-cell patch clamp technique, the currents of the transient outward potassium channel were recorded off-line, and then the effects of EMF on the channels were investigated. Compared to the control group, a significant inhibition on the I_A was reported, and the rate of inhibition were (63.0 ± 2.2)% (1 mT), (55.0 ± 1.7)% (5 mT), (38.0 ± 1.8)% (10 mT), respectively. Moreover, the characteristics of activation and inactivation were both influenced by EMF, because there were a decreasing both on the half activation voltage and the half inactivation voltage. Additionally, different effects on the channels had been found for the different intensities. In the research, the maximum inhibition rate of current was induced by 1 mT, and the changing of the half activation voltage and the half inactivation voltage were the biggest after exposure to 5 mT, and 10 mT EMF increased the slope factor of inactivation. The results indicated that EMF affected the conformational changes of ion channel protein on cell membrane, and further influenced the normal function of ion channels.