Duodenal neuroendocrine tumors (DNETs) are rare tumors that are occasionally found during upper endoscopies. The incidence of DNETs is increasing, although the data regarding treatment outcomes are insufficient. The aim of this study was to evaluate the treatment outcomes in patients with nonampullary DNETs who underwent endoscopic resection or surgery. We evaluated the medical records of patients who were diagnosed with nonampullary DNETs from 2004 to 2017 in 7 university hospitals. We retrospectively analyzed clinical characteristics and compared therapeutic outcomes based on the endoscopic lesion size and treatment method. We ultimately enrolled 60 patients with nonampullary DNETs who underwent endoscopic and surgical treatments. In the endoscopic treatment group, the en bloc resection, endoscopic complete resection (CR) and pathologic CR rates were 88%, 92%, and 50%, respectively. The endoscopic treatment group was divided into 3 subgroups based on the lesion size (1-5 mm, 6-10 mm, and ≥11 mm). The pathologic CR rate was significantly lower in the subgroup with a lesion size ≥11 mm (0%, P = .003) than those in the other 2 subgroups. Lymphovascular invasion occurred significantly more frequently (33.3%, P = .043) among those with a lesion size ≥11 mm. The pathologic CR rate in the surgical treatment group was higher (90.9%) than that in the endoscopic treatment group (50%, P = .017). Surgical treatment appears to be a more appropriate choice because of the risks of incomplete resection and lymphovascular invasion after endoscopic treatment for lesions larger than 11 mm.