The incidence of colorectal cancer in China is increasing year by year. Studies have shown that colorectal cancer is more common in the left colon，especially in the China. Our previous study also showed a higher rate of missed adenoma in the left colon than the right colon during colonoscopy. Additionally, prolonging withdrawal time could only improve the ADR of right colon, but had limited effect on the ADR of left colon in our previous research. Our aim is to evaluate the effect of a second forward view examination of the left colon on the detection of adenoma detection during colonoscopy.

Inclusion Criteria:

• Patients whose age are between 40-75, or aged 76-85 depending on his condition
• Patients who have indications for screening
• Patients who have signed inform consent form.

Exclusion Criteria:

• Patients who have undergone colonic resection
• Patients with alarming signs and symptoms of colorectal cancer: hematochezia, melena, anemia, weight loss, abdominal mass, positive digital rectal examination
• Patients with abnormal blood coagulation or taking antiplatelets or anticoagulants within 7 days before colonoscopy
• Patients with inflammatory bowel diseases
• Patients with a history of abdominal surgery, or highly suspected or confirmed colorectal cancers by radiographic and laboratory tests Patients with hereditary colorectal cancer syndrome (including familial adenomatous polyposis).
• Patients with pregnancy, severe chronic cardiopulmonary and renal disease.
• Patients with failed cecal intubation
• Patients with poor BPQ that necessitated a second bowel preparation
• Patients with therapeutic colonoscopy for existing lesions
• Patients refusing to participate or to provide informed consent