Background: Carbon-ion radiotherapy (CIRT) is a unique external radiotherapy that has an excellent dose conformity and higher biological effectiveness compared to conventional radiotherapy. The sensitivity of the organs in upper abdomen has limited the radiation dose to levels that are ineffective against pancreatic cancer, which has extreme radioresisnatnce. We investigated effectiveness and gastrointestinal (GI) toxicity of CIRT. Methods: From April 2014 to June 2015, 41 patients with pancreatic cancer were treated with definitive CIRT. 26 patients who were confirmed as unresectable locally advanced pancreatic cancer pathologically and by imaging techniques were included in this retrospective analysis. 15 patients were excluded because of resectable disease (n = 3), borderline disease (n = 4), no cell evidence (n = 11). CIRT was performed with 55.2 Gy (RBE) at 12 fractions. The maximal absolute dose with covered 2cm³ of the organ (D2) was restricted under 46 Gy (RBE). All patients were followed up using CT, FDG-PET and upper gastrointestinal endoscopy at least every six months. Anti-tumor effect was evaluated using CT and FDG-PET according to RECIST. If FDG accumulation was negative or equal to normal pancreatic tissue after CIRT, it was defined CR even no decreasing in seize on CT. GI toxicity was evaluated using CTCAE. Results: Tumor location were head in 7, head-body in 2, body-tail in 16. Median tumor size was 35mm (range 21-50). In all patients, sequential and/or concurrent chemotherapy was performed. The median follow-up period was 7.7 months. Local effect using CT criteria was CR in 0, PR in 2, SD in 22, and PD in 2. The evaluation adding FDG-PET criteria was CR in 12, PR in 4, SD in 8, and PD in 2. Two patients who were SD by CT and CR by FDG-PET performed tumor resection and were revealed pCR. The actual incidence of gastric ulcer/bleeding was 15% (n = 4). Only one patient (4%) received blood transfusion because of grade 3 gastric ulcer. The other patients experienced grade 1/2 gastric ulcer. There was no duodenal ulcer. Conclusions: CIRT were effective and well tolerated under the restriction of D2 < 46 Gy (RBE). FDG-PET was useful for the evaluation of local effect after CIRT.