Objective To evaluate the characteristics of EMG on bilateral extremities tonic seizures in patients with localization-related epilepsy. Methods A total of 74 seizures in 22 localization-related epilepsy patients were included in observed group. 57 seizures of 6 patients who had generalized epileptic discharge were included in control group. After quantitative and qualitative analysis of the EMG，the positive rate of lateralizationand its significance were evaluated. Results During the typical tonic phase of seizures，the motor unit action potential of muscles appeared in the form of build-up. The initial EMG activities in 83.8%（62/74）seizures were of lateralizing signs to the side contralateral to the seizure onset. In observed group，the build-up duration ofinitial tonic EMG activities was 3 s and the difference of bilateral tonic EMG activities was 2.44 s. While in control group，it was 0.89 s and 0.14 s. There were statistically significant differences between two groups. In observed group，the positive rate of lateralization based on BETS’s EMG in the lateral cortex epilepsy group was higher than that in parasagittal cortex epilepsy group （88.2% vs. 73.9%）. Conclusions The asymmetric EMGis an objective and sensitive lateralizing sign. When the difference of bilateral tonic EMG activities is longer than 250 ms，it is possible that the epilepsy has a focal origin.