Colorectal cancer is one of the most common cancers. Worldwide, PET/CT has been applied in diagnosis and staging, evaluating recurrence and monitoring treatment for patients with colorectal cancer. PET/CT has been first operated in Vietnam since 2009 and till now, there are no studies that evaluate the value of PET/ CT in the diagnosis of primary tumor status in colorectal cancer. Patients and Methods: 97 patients with colorectal cancer were using PET/CT for diagnosis and staging at The Nuclear medicine and Oncology Center, Bach Mai Hospital from August 2009 to July 2011. Results: The average age was 55.9. Male/female is 1.55/1. Colon cancer is more common than rectal cancer (56 vs 41). In the study, PET/CT detected 36 patients with 15 lesions in colon and 21 in rectum. Sensitivity of PET/CT in detecting primary tumor is lower than the colonoscopy in colon but higher in rectum, especially in recurrent cases. In comparison to CT, PET/CT is superior in finding lesions and evaluating tumor invasion in colon (88.2 percent 17.6 percent vs) and rectum (100 percent vs 42.8 percent). Average size of tumors are: colon (5.55:1: 3.39) cm and rectum (5.70:1: 2.91 cm). With PET/CT results, the number of patients with stage IV was increased. The percentage of patients increased tumor stage (T) is 5/36 (13.9 percent). Conclusions: Application of PET/CT in the diagnosis primary tumor for colorectal cancer patients has a high benefit. Colorectal cancers should being used both colonoscopy and PETICT to provide accurate stage of the tumor in oder to select the most appropriate treatments invidually.