The prophylactic Argon plasma coagulation (APC) of non-bleeding visible vessels seems not to have an additional advantage in the prevention of delayed post polypectomy bleeding in recently studies. However, immediate APC after endoscopic mucosal resection (EMR) may not mean the complete coagulation of exposed vessels because of injection material. The aim of this study was to evaluate the clinical feature of the exposed vessels in ulcer base over time after prophylactic APC in colonic EMR. Methods: This study was designed as a prospective study. Investigators excluded diminutive polyp, large polyp (> 2cm), long stalk pedunculated polyp, and clear ulcer base after EMR. Between August 2013 and May 2014, patients who were enrolled underwent prophylactic APC for non-bleeding visible vessels after colonic endoscopic mucosal resection. After checking the complete coagulation of visible vessels, the numbers of visible vessel group were counted by lap of time such as 1, 3, 5, and 7 min in ulcer base.