The investigators prospectively enroll 130 adults that receive colonoscopy examination from Jan 1, 2014 to June 30, 2014. Written Inform consents are achieved before colonoscopy. All patients are randomly assigned to group A and group B. The group A is named as "the setting of colon cleansing room group". The group B is named as " the standard colon preparation group". The randomization was done by a study nurse who is well trained about the teaching course of bowel cleansing level evaluation, low residual diet education and preparation method. Demographic data is collected through questionnaire before colonoscopy. Diet restriction for two days with lower residual diet is well educated through a detail checklist. All patients receive a brief counselling session. The investigators outline the colon preparation instructions. The investigators also explain the rationale for colon preparation with a complete checklist and the picture of colon cleansing assessment. All patients in group B initiate the standard colon preparation with PEG-ELS made from two sachets of PEG (Klean-PrepTM, Norgine Ltd. Harefield, Middlesex, United Kingdom) dissolved in 2 L of water at 8:00 AM on the day of colonoscopy. All patients in group A drink two liter PEG-ELS in the colon cleansing room which is a in-hospital setting for colon preparation at 8:00 AM on the day of colonoscopy. The time interval in both group between colon preparation and colonoscopy is 3 to 5 hours. All colonoscopies are performed by two endoscopists in Jin-Shan Branch Hospital with an Olympus CF-260AL colonoscope (Olympus Optical Co. Ltd., Tokyo, Japan). All endoscopists were trained at National Taiwan University Hospital with at least eight hundred colonoscopy experience. The endoscopist follow the standard protocol to insert, withdraw and observation. Intramuscular injection of hyoscine butylbromide is done as antispasmodic if there is no contraindication. Complete colonoscopy is defined as reaching and taking a picture of the cecum. Endoscopists take pictures at every anatomical portion of colon including cecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum before and after fecal material is washed out. Theendoscopist also record all of the colonoscopy examination as video files. All colonoscopists are asked to take at least thirty pictures during whole colonoscopy examination. Colon cleansing quality is scored by Boston Bowel Preparation Quality Scale. The scale is applied to each colon segment including right colon (R), middle colon (T) and the rectosigmoid colon(L). The score is calculated by adding the 0 to 3 ratings for each colon section. Electronic files of endoscopic pictures without lesion presentation and edited videos were reviewed by all endoscopist. All the detected adenomas during both study group are recorded and stored as electronic files and video including location, size, morphology and endoscopic diagnosis. The primary endpoint of study is to compare the Boston Bowel Preparation Quality Scale between the two study group. The secondary endpoint is to compare the average number of adenoma detection between the two study group. Sample size is estimated by the average number of adenoma detected (1.0 lesions per subject in standard preparation group, SD 1.0 and 1.5 lesions per subject in hospital preparation group, SD 1.0) according to a previous study in National Taiwan University Hospital and the pilot study in National Taiwan University Hospital , Jin Shan Branch , more than 80% power by enrolling 65 subjects in each group are achieved ( t test, α error of 0.05).