Abstract: PET-CT is a good modality for localization of the malignant lesions. It is superior to PET in the diagnosis, localization and guiding treatment for head and neck cancer. Possessing of the superiority of PET and CT, PET-CT can improves the accuracy and confidence for diagnosis and staging and helps to reliably distinguish malignant lesions from physiologic uptake and some inflammation. It is also helpful for guiding more accurate radiotherapy. Although PET-CT has a lot of superiorities, it still has some shortcomings in the diagnosis of the thyroid cancer, parotid gland tumor and in the discriminating malignancy from some active inflammation.