Alopecia areata is a chronic, organ-specific autoimmune disease characterized by hair loss in localized or diffuse pattern on hair-bearing areas including scalp. The course of alopecia areata is unpredictable but most patients have recurred in more than one episodes of hair loss. We herein report a case of alopecia areata developed after the diagnosis of thymoma, which are known to be associated with paraneoplastic autoimmune diseases at a high frequency. In this case, the patient’s hair loss recovered rapidly after thymectomy with applying topical and systemic treatment for six months. And this was faster than the recovery rate of other alopecia areata. We therefore considered this case to be alopecia areata associated with thymoma and focused on the possibility that this hair loss has improved after thymomectomy. So we presented the very rare case of alopecia areata in a patient associated with myasthenia gravis-free thymoma.