Aim: Assessing the value of PET/CT in staging diagnosis of colorectal cancer. Patients and Methods: 134 colorectal cancer patients have performed PET/CT images at the Nuclear Medicine and Oncology Center, Bach Mai Hospital from August 2009 to Febrary 2012. Results: Average age: 56.13. Male/female: 1.62/1. In the study, PET/CT detected 100 percent patients with 40.9 percent lesions in colon and 59.1 percent tumors in rectum with SUV max highly. Average size of tumors by location are 5.78 + or - 3.12 cm and 5.86 + or - 3.07 cm in colon and rectum, respectively. Comparison with CT: PET/CT detected more lesions of metastases espescially in bone and lymphatic nodes. The average SUV max values in the metastasis lesions. 20.9 percent patients changed stage after doing PET/CT. Conclusions: PET/CT is highly efective in diagnosis of primary tumor and helps physicians giving the optimal treatments for colorectal cancer patients.