Objective: Assesing the PET/CT role in diagnosis ofunknown primary tumor. Patients and methods: Retrospective and prospective description study with histologically or cytologically proven mestasteses 48 patients of unknown primary tumor (UPT), they were being performed PET/CT at Nuclear Medicine And Oncology Center, Bach Mai Hospital between september 2009 and december 2013. Results: The detected rate of primary tumor by PET/CT was 66,7 percent(32/48) and the undetected rate of primary tumor by PET/CT was 33,3 percent(16/48). The primary tumors were identified mostly in the lung 26 percent(n=8), followed by stomach 16 percent (n=S). In this study, PET/CT detected additionally 99 metastatic sites compared with 48 positions before. Conclusion: PET/CT is a valuable diagnostic method in detecting unknown primary tumor.