Lung cancer is the main cause of cancer deaths in the world and positron emission tomography (PET) is considered as the most accurate diagnosis and staging technique for lung cancer. For human cancers, fluorodeoxyglucose (FDG)-PET imaging of most primary and metastatic tumors will show significantly increased glucose uptake because high metabolic activity of cancer cells. But there still have the question of false negative or positive rates in diagnostic accuracy need to be considered.A 51 year old man was diagnosed a lung tumor in the right middle lobe without enlargement of lymph nodes by computed tomography (CT). The F-FDG-PET/CT presented 1 slight increased metabolism in the tumor region. After resection of the tumor, postoperative pathological examination confirmed that it was invasive lung adenocarcinoma and with visceral pleural invasion while showed relative low glucose absorption in PET/CT.A form of invasive lung adenocarcinoma was diagnosized. The tumor tissues were further confirmed by immunohistochemical assessments, which showed that thyroid transcription factor 1 (TTF-1 or NKX2-1) and Cytokeratin 7 (CK7) were all significant positive.Diagnosis of lung cancer even all other cancers by FDG-PET should be carefully considered the question of accuracy. Our case has added additional literature for us to considering the false-negative of lung cancer diagnosis by F-FDG-PET/CT.