Seven patients with rectal cancers which were difficult to ensure surgical margins for because of huge tumors(over 60mm in diameter), invasion to other organs, or severe nodal metastases, were treated with preoperative chemotherapy consisting of 2-10 courses of mFOLFOX6.The response rate was 85. 7%.Complete response was observed in one patient, and partial response was observed in 5 patients.Four to 5 weeks after chemotherapy, surgery was performed for all patients.Following surgical procedures, abdominoperineal resections were performed in 4 cases, low anterior resections in 3 cases, and removal of the liver metastases(not diagnosed preoperatively)in 2 cases.R0 resections were also performed in all patients. According to the histological regression grading of the resected specimens, one patient had a complete disappearance of tumor, 5 had grade 1a regression, and one had grade 1b regression.One of the 7 patients had recurrence at the lung. However, another patient survived without recurrence. In this study, preoperative mFOLFOX6 chemotherapy was expected to be an effective treatment for improving the curative resection rate of patients with tumors which were difficult to ensure surgical margins because of huge tumors, invasion to other organs, or severe nodal metastases.