Background and Study Aims: For one-piece resection the conventional technique of endoscopic mucosal resection (EMR) is limited to gastric mucosal tumors of 10 mm or less in size. In this retrospective study, we investigated the efficacy and complications associated with a new EMR method, using an insulated-tip diathermic knife (IT-EMR). Patients and Methods: In a total of 41 patients gastric mucosal tumors were resected using IT-EMR. Results: One-piece resection rates were 82 % (14/17) for lesions of 10 mm or less, 75 % (12/16) for those between 11 and 20 mm, and 14 % (1/7) for those of over 20 mm. Complication rates for severe bleeding and perforation were 22 % and 5 %, respectively. With a median follow-up period of 32 months, no recurrence was observed after these procedures. Conclusions: Compared with conventional EMR, this new method may have significant benefits, particularly regarding one-piece resection of lesions between 11 and 20 mm in size, and may also have a lower recurrence rate.