Abstract: Objective The objective of our study is to assess the roles of 18F-FDG-PET/CT in the diagnosis of hepatic primary and metastatic malignant lesions, especially in different size lesions.Materials and Methods 217 patients with hepatic malignant lesions who had undergone whole body 18F-FDG-PET/CT were retrospectively reviewed, including 120 patients with primary hepatic malignant tumors, 97patients with metastatic hepatic carcinoma.38 patients with primary hepatocelluar carcinoma had undergone liver three phases contrast enhance by MSCT after whole body 18F-FDG-PET/CT scanned.We compared with the SUVmax between primary and metastatic lesions with different sizes, and carried out the survival analysis between primary and metastatic small lesions.PET images were divides three grades according to tracer intense.We calculated the sensitivity in primary and metastatic lesions with different images grades, then combined with MSCT contrast enhance images to analyze.Results 75 patients with primary hepatocelluar carcinoma with 1 grade images are 24 patients (32%) , 23 grades are 51 patients (68%) .93 patients with Metastatic liver carcinoma with 1 grade images are 4 patients (4.3%) , 23 grades are 89 patients (95.7%) .the sensitivity of PET plus MSCT in primary small hepatocellular carcinoma is 37.5% and 73.7% in big hepatocellular carcinoma.There are different in lesions size of SUVmax and in two type whose lesions size is smaller than 3cm, without different in two type whose lesions size is bigger than 3cm.survival analysis indicated that survival rate is no different between SUVmax bigger than 2.5 and SUVmax smaller than 2.5 groups.Conclusion The sensitivity of 18F-FDG PET/CT is low in primary hepatocelluar carcinoma, but combined with contrast enhanced CT can improve the sensitivity.The sensitivity of 18F-FDG PET/CT is high in metastatic malignant tumors.