Description: The application of stereotactic ablative radiotherapy (SABR) to hepatocellular carcinoma (HCC) is emerging. To identify pretreatment prognostic indicators is crucial for patient selection and optimal individual therapy. The aim of this study was to determine whether 18 F-FDG PET and a combined 18 F-FDG–contrast CT parameter could be useful tools to predict tumor control for patients with HCC treated by SABR. Methods: We retrospectively identified 31 patients (41 tumors) who underwent 18 F-FDG PET before SABR between November 2007 and September 2011. 18 F-FDG PET parameters were collected as prognostic indicators, including visual PET scale (+/–), maximal standardized uptake value (SUV) of the tumor (T SUVmax ), ratio of T SUVmax to maximal normal-liver SUV, ratio of T SUVmax to mean normal-liver SUV, and score combining tumor volume and T SUVmax (CT/ 18 F-FDG PET score). They underwent SABR with a median dose of 42 Gy (ranging from 30 to 50 Gy) in 4–5 fractions. 18 F-FDG PET parameters and clinical factors were tested as predictors of tumor control and patient survival. Results: The median follow-up time was 18 mo. Among the parameters examined, T SUVmax and CT/ 18 F-FDG PET score were significantly correlated with tumor control. T SUVmax with a cutoff value of 3.2 was the most significant prognostic indicator. The 4-y control rate was 86.2% in tumors with a T SUVmax of 3.2 or less but only 37.5% in those with a T SUVmax of more than 3.2 (adjusted hazard ratio, 9.40; 95% confidence interval, 1.18–74.76; P = 0.034). CT/ 18 F-FDG PET score (≤4 vs. 〉4) was also a significant predictor of tumor control after SABR. Tumors with a CT/ 18 F-FDG PET score of more than 4 had a 5.23-fold risk of tumor failure. After adjustment for factors of sex, American Joint Committee on Cancer stage, Cancer of the Liver Italian Program score, and Child–Pugh classification, tumors with a score of more than 4 had a 4.96-fold risk of failure after SABR, compared with tumors with a score of 4 or less. For overall survival, none was statistically significant. Conclusion: The use of 18 F FDG PET to predict tumor control is feasible. T SUVmax with a cutoff value of 3.2 is the best prognostic indicator. We suggest that 18 F-FDG PET may be a reference for prognostic prediction, patient selection, and radiation dose adjustment for HCC patients treated with SABR.