Background: hepatocellular carcinoma (HCC) is the fifth most common tumor worldwide and its incidence is increasing especially in western nations. HCC may be associated with portal vein thrombosis which could be either benign or malignant. Aim of the Work: to detect the role of diffusion MR imaging in the differentiation between benign and malignant portal vein thrombosis in patients with HCC. Materials and Methods: prospective study carried on 30 patients with liver cirrhosis with pathological or radiological proof of HCC, associated with visible portal vein thrombosis. Dynamic MRI examination and diffusion weighted imaging were performed for all patients. ADC values and ratios were calculated for both HCC and thrombus. Definite accepted radiological criteria were used as a standard of reference to detect the nature of the thrombus and discriminate between bland ones and neoplastic ones. Results: we found that the ratio between the ADC of the thrombus and ADC of the HCC was significantly different between the malignant and bland cohorts and a cutoff value of 1.25 for the ADC ratio helped in differentiation between malignant and benign portal vein thrombi with 85% sensitivity and 81% specificity. There was also obvious statistical significant difference between the ADC values of the bland and neoplastic thrombi. Conclusion: diffusion weighted MR imaging has a great role in differentiation between the benign and malignant portal vein thrombosis by measuring the ADC ratio between the thrombus and HCC.