Patients with hepatocellular carcinoma (HCC) often have liver volume (LV) reduction caused by cirrhosis and are frequently ineligible for ablative photon radiotherapy. This study is to report the outcomes in HCC patients with small LV treated with high-dose hypofractionated proton beam therapy (PBT) and to elucidate the dosimetric advantages of protons versus photons. Sixty-two HCC patients with pretreatment LV < 1000 ml undergoing curative-intent PBT (with unirradiated LV to standard LV ratios [ULV/SLV] >30% for Child-Pugh [CP] class A and >40% for CP class B ) were included in this study. The most common dose-fractionation schedules were 72.6 GyE/22 fx. for central tumors (≤1 cm of gastrointestinal tract and porta hepatis [PH], N = 42) and 66 GyE/10 fx. for peripheral tumors (>1 cm from gastrointestinal tract/PH, N = 15). Photon treatment plans was performed in all patients using the same dose schedules as those of PBT, and the dosimetric parameters were compared. Additional photon plans with 36 Gy/3 fx were performed to investigate the eligibility of stereotactic body radiotherapy (SBRT) based on the safety threshold that >700 ml of LV receiving ≤15 Gy. The median pretreatment liver volume was 829 ml (range, 595-995 ml), whereas the median tumor size was 6.0 cm (range, 0.8-22.7 cm). After a median follow-up of 12 months, the 1-year LC, PFS, and OS in patients treated with definitive PBT were 96%, 57%, and 86%, respectively. Radiation-induced liver disease (RILD) was recorded in 3 (4.8%) patients. In the treatment plan comparison using the same hypofractionated dose schedules, proton plans were associated with significantly lower mean doses of liver (16.5 vs. 22.3 Gy, P <0.001), stomach (2.4 vs. 8.4 Gy, P <0.001), duodenum (4.3 vs. 7.2 Gy, P <0.001), lower third esophagus (4.6 vs. 13.9 Gy, P <0.001), heart (3.7 vs. 6.7 Gy, P <0.001), left kidney (2.3 vs. 2.9 Gy, P <0.001), spinal cord (from T9 to L3, 4.5 vs. 7.4, P <0.001), and body (from T9 to L3, 6.4 vs. 9.5, P <0.001), mean D2cc of stomach (9.9 vs. 20.7 Gy, P <0.001), duodenum (16.5 vs. 21.3 Gy, P = 0.002), lower third esophagus (10.8 vs. 22.3 Gy, P <0.001), and mean maximum dose of spinal cord (15.3 vs. 22.7 Gy, P <0.001), compared with the photon plans. As converting the dose-volume histogram (DVH) dose-bins to equivalent doses in 2 Gy (EQD2), 21 (34%) patients had mean liver dose >28 Gy in their photon plans and thereof necessitated target dose de-escalation. In the SBRT plans with 36 Gy in 3 fractions, 50 (81%) patients failed to comply with the constraint that >700 ml of LV receiving ≤15 Gy. The majority of HCC patients with LV <1000 ml were ineligible to SBRT and frequently required dose compromization for hypofractionated photon radiotherapy. Strikingly, PBT permitted these patients to undergo ablative radiation treatment and offered substantial local control benefits with <5% of RILD risk.