We studied 240 cases of unresected hepatocellular carcinoma (HCC) using Cox's proportional hazard model to elucidate which factors would be closely related with the survival period after treatment by transcatheter arterial embolization (TAE) in the presence or absence of iodized oil. The results were as follows. The cumulative survival values obtained after TAE were 67.5% for 1 year, 32.0% for 2 years, and 20.5% for 3 years. The most significant prognostic factor was the degree of extension of tumor embolus in the portal vein or its branch. The tumor extension and the tumor type were also important factors. Age, sex, and AFP, HBsAg, and HCV Ab values were not useful as prognostic factors. This study provides a rational background for the selection of treatment for HCC. Furthermore, knowledge of the prognostic factors is useful for the management of patients, particularly in maintaining their good quality of life.