Abstract: Twenty-four gastric cancer specimens with preoperative radiatherapy were used in this work to study the histological changes produced by radiation. All cases were diagnosed as gastric cancer by endoscopic biopsies before irradiation, and their slides were compared with those of resected specimens. The radiation effect were divided histopathologically into three degrees: slight, moderate and severe. The moderate and severe degrees were regarded as effective and the slight degree was regarded as noneffective. In this series of 15 cases who have received 3500—4000 rad, all except one showed effectiveness without obvious injury to the surrounding tissue. In the irradiation area all of the cancer cells, no matter how deeply it invaded and how far it metastasized, showed evidence of effectiveness, more or less, of irradiation. Thus, this study indicates that preoperative radiotherapy is benificial to the prognosis in patients suffering from gastric cancer and 3500—4000 rad may be the proper dosage.