The accurate evaluation of response to treatment is a key aspect in cancer therapy, because an objective response may become a surrogate marker of improver survival. For cytotoxic drugs, tumor response evaluation according to the World Health Organization (WHO) criteria or the Response Evaluation Criteria in Solid Tumors (RECIST) guideline offers simple approaches based on the size of the lesions. However, considering the nature of locoregional therapy or new cytostatic agent for hepatocellular carcinoma (HCC), which do not decrease the size of the tumor but induce tumor necrosis, original WHO or RECIST criteria will not reflect clinical benefit exactly. Recently, modified RECIST assessment is proposed by AASLD-JNCI guidelines. Given that complete necrosis was well correlated with better survival, modified RECIST criteria consider changes in tumor viability, which can be measured as the area of arterial enhancement, with maintaining overall response assessment similar to RECIST. The proposed modified RECIST assessment is expected to provide a reliable method for assessing tumor response in HCC clinical trials.