Abstract: Objective To investigate therapeutic efficacy of cyclophosphamide on thymoma in patients with myasthenia gravis. Methods From January,2010 to December,2015,93 cases of myasthenia gravis with thymoma in Huzhou Central Hospital and Second Affiliated Hospital were selected and divided into low malignant thymoma group(44 cases) and high malignant thymoma group(49 cases) according to the degree of thymoma malignancy.The patients in both groups received cyclophosphamide intravenous treatment.The general condition,the quantitative myasthenia gravis score,therapeutic efficacy and adverse reactions were compared between the two groups. Results The quantitative myasthenia gravis score between the two groups had no differences before and after the treatment(P>0.05).After the treatment,the quantitative myasthenia gravis score in both group L and group H were decreased(P<0.05).The curative effective rate in group L was 88.6% and in group H was 85.7%,the difference was not significant(P>0.05).There was no difference in the white blood cell count dropping,aminotransferases increasing,fatigue,nausea and vomiting between the two groups(P>0.05). Conclusion Cyclophosphamide is safe and effective in the treatment of thymoma in patients with myasthenia gravis with,the therapeutic efficacy is similar for low and high malignant thymoma.