Inflammatory myofibroblastic tumor(IMT) or inflammatory pseudotumor is a rare, solid tumor that most often affects children. This tumor is characterized by a spindle cell proliferation admixed with a variety of inflammatory cells. Although it has disputed nosology, a distinctive fibroinflammatory and even pseudosarcomatous appearance have been well appreciated. Herein, we report a case of IMT in the posterior mediastinum in a 19-year-old girl with clinical findings. The immunohistochemical and ultrastructural studies on the tumor cells are reported, and their distinctive characteristics are discussed in details.