Background: Thymoma is the most common tumor of the anterior mediastinum that has the most effective treatment, as it can be completely resected. In patients with advanced stage, phrenic nerve involvement can be seen and suggested treatment for these patients is unilateral phrenic excision and diaphragm plication. However in patients with myasthenia gravis, there are concerns in relation to this method of treatment. The aim of this study is to evaluate the effects of plication of the diaphragm on complications of phrenic nerve excision in thymoma patients with and without myasthenia gravis involving the phrenic nerve. Materials and Methods: A retrospective cohort study was performed on 26 patients with thymoma; half of the patients had myasthenia gravis and the other half did not have myasthenia gravis. We performed diaphragm plication in 7 patients in each group with excision of phrenic nerve. Patients were evaluated based on preoperative and postoperative variables. Results: The patients’ age (P=0.943), sex (P=0.999), blood loss during surgery (P=0.919), need for transfusion during surgery (P=0.999), short term complications (P=0.186), need for tracheostomy (P=0.27) and mortality (P=0.09) differences were not significant. However, the average duration of ICU stay (P=0.001) and intubation in ICU (P=0.001) in patients who had myasthenia gravis was more than patients without myasthenia gravis. These values were less in patients with myasthenia gravis and diaphragm plication than patients with myasthenia gravis and no diaphragm plication. Conclusion: Excision of the phrenic nerve in patients with myasthenia gravis associated with thymoma and phrenic nerve involvement is appropriate.