Inclusion Criteria:  Adults 50-75 years old  Exclusion Criteria:  People with history of intestinal surgery; People with history of colorectal cancer; People with history of inflammatory bowel disease, ischemic enteritis, vascular malformation of intestine or other disease resulting in intestinal tract bleeding; People with symptoms including visible rectal bleeding, hematuria, severe and acute diarrhea; Pregnancy, lactation or menstrual phase; Severe congestive heart failure or other sever disease cause cannot tolerate colonoscopy.