We retrospectively evaluated clinical factors affecting long-term survival after treatment for hepatocellular carcinoma (HCC) to predict the reliability of the prognosis of patients with HCC. We analyzed 157 patients who received treatment for HCC. The prognostic index (PI) was evaluated using the Cox regression model. The overall cumulative survival rates were 79.7% at 1 year, 51.1% at 3 years, and 24.9% at 5 years. The PI was calculated by the following formula, consisting of five factors: PI = 0.637 x number of tumor lesions + 0.103 x tumor diameter + 1.003 x ascites + 0.915 x portal vein tumor thrombosis - 0.006 x cholinesterase + 2.0. It was found that liver function and progression of the tumor are the most important factors for prognosis after treatment for HCC. The PI, based on five factors, will in future be an appropriate index to predict the prognosis of patients with HCC.