Objective To assess the utility of single fiber EMG (SFEMG)in the differential diagnosis of hyperthyroidism associated with ocular myasthenia gravis (OMG)and Graves ophthalmopathy.Methods SFEMG was performed in orbicularis oculi muscle of 2 groups of patients.including 32 patients with hyperthyroidism associated with OMG,and 35 patients with Graves ophthalmopathy.The parameters of SFEMG between different groups were compared.Results The mean jitter was (96.2 ± 23.7),(42.8 ± 12.6)μs in hyperthyroidism associated with OMG and patients with Graves Orbitopathy. M50of the percentage of jitter ＞55 μs was 92% and 5% in the 2 groups respectively.M50of the percentage of block was 25% and 0 in the 2 groups respectively.Fiber density was (1.9 ± 0.4)and (1.7 ± 0.5)in the 2 groups respectively.There was significant difference in those parameters of SFEMG between the patients in 2 groups(t＝15.56,Z＝9.26,Z＝7.35,all P＜0.01).Conclusion SFEMG of orbicularis oculi muscle shows significantly increased jitter and block in hyperthyroidism associated with OMG,which can help to differentiate hyperthyroidism associated with OMG from Graves ophthalmopathy.