Objective: To explore the characteristics of clinicopathology and diagnostic criteria about primary mediastinal large B-cell lymphoma (PMBL). Methods: The clinical features, histological and immunohistochemical features of these cases were analyzed, the related literatures were reviewed. Results: Of the 3 patients with PMBL, two were males and one was female. In all the cases adjacent organs were invaded. Cervical or supraclavicular lymph node were involved in two cases, while vertebral were involved in one case. Histologically the neoplasm cells infiltrated nested or diffusely, with varying degrees of fibrosis. The tumor cells were medium to large size with abundant pale cytoplasm and round or oval nucleoli. Necrosis was seen in one case. Immunohistochemically, CD20, CD79a, CD30, CD23 and bcl2 were positive, while CD3 and CD5 were negative in all three cases, CD30 were positive in two cases. Two patients achieved PR, the other achieved CR. Conclusion: PMBL is a rare non- Hodgkin lymphoma, and the morphology can be variable and confusing, sometimes it could be misdiagnosed. To avoid misdiagnosis, it is important for us to be fully aware of this tumor.