A 51-year-old female patient was admitted to our hospital with a diagnosis of sigmoid colon cancer, and a sigmoidectomy and D3 lymphadenectomy was performed. The tumor was a moderately differentiated tubular adenocarcinoma (pT3N0M0, pStage II). No adjuvant therapy was administered. An abdominal computed tomography (CT) performed 11 months after the sigmoidectomy revealed a hepatic metastasis. The patient chose to undergo carbon ion radiotherapy (53.0 GyE/1 Fr). However, a second CT performed 10 months later indicated regrowth of the hepatic metastasis. A partial liver resection was performed, and severe deformity was observed in the irradiated region of the liver. The borderline between the tumor and surrounding tissue was not clearly discriminated. Histopathological examinations indicated increased numbers of fibrous cells in the irradiated region. To perform hepatic resection safely for liver metastasis of colorectal cancer following heavy particle radiotherapy, it is necessary to accurately determine the resection line before and during surgery. When feasible, resection of the whole irradiated area should be considered.