Objective: To investigate the demographics and diagnostic yield in a cohort of Chinese pediatric patients undergoing colonoscopy in one institution over 12 years. Methouds: The study participants were consecutive patients aged <18 years that underwent their first colonoscopy in the endoscopy center at Peking University Third Hospital between Jan. 1, 2005 and Dec. 31, 2017. Demographic, endoscopic, and pathological findings were collected. According to the age of the patients, they were divided into 0-3 year-old group, 4-6 year-old group, 7-14 year-old group and 15-17 year-old group. The patients were also divided into 2005-2011 group and 2012-2017 group, according to the time of colonoscopy. Results: The cohort consisted of 326 patients, including 205 boys (62.9%) and 121 girls (37.1%). In the study, 31 patients (9.5%) were in 0-3 year-old group, 28 (8.6%) were in 4-6 year-old group, 96 (29.4%) were in 7-14 year-old group and 171 (52.5%) in 15-17 year-old group. The terminal ileum intubation success rate was 90.5% (295/326). No serious complications such as hemorrhage or perforation occurred during the procedures. The cleaning effect was good in 92.3% (301/326) of the patients. A total of 204 patients (62.6%) received a positive diagnosis under colonoscopy. 27.0% (88/326) of the patients was diagnosed as nonspecific colitis or terminal ileitis. 46 (14.1%) with inflammatory bowel disease (IBD) and 39 (12.0%) with polyp. The diseases were significantly different among the different age groups. The highest IBD diagnostic rate was found in 0-3 year-old group (7/31, 22.5%), while the highest polyp finding rate was in 4-6 year-old group (8/28, 28.6%). The number of the patients in 0-3 year-old group was significantly increasing in 2012-2017 group compared with 2005-2011 group (27/191 vs. 4/135, P=0.001), while the terminal ileum intubation success rate was higher (179/191 vs. 116/135, P=0.037). However, comparisons between years 2005-2011 and 2012-2017 showed that neither IBD nor polyp detection rate changed significantly (P=0.850). Conclusion: Colonoscopy in pediatric patients was a safe and effective procedure. Colitis or terminal ileitis was the primary finding during colonoscopy while IBD was the second one, and polyp was the third. However, the diagnostic yield did not change significantly. IBD was not as quickly increased in our hospital as it was in South China.