We investigated whether peppermint oil (PO) is useful for endoscopic diagnosis of gastric tumors without magnifying endoscopy. Twenty-six patients diagnosed with gastric tumors were examined. Endoscopic images were recorded by white light (WL) and narrow band imaging (NBI) without magnifying endoscopy. After PO administration, images were recorded again by WL and NBI (PO+WL and PO+NBI). The clarity of tumor margins and the effect of PO on the clarity of tumor margins were scored. The mean scores for clarity of tumor margins were 3.6 points in WL, 4 in NBI, 4.3 in PO+WL, and 4.3 in PO+NBI. The scores of WL and NBI showed a tendency to rise by PO. The mean scores for the effect of PO on the clarity of tumor margins were 2.7 in WL and 2.5 in NBI. Therefore, we conclude that PO is useful for endoscopic diagnosis of the margin of gastric tumors.