Inclusion Criteria:  50-75 years old people; People sign an "informed consent form"  Exclusion Criteria:  People with history of colorectal surgery; People with history of colorectal cancer; People with history of other diseases that may produce fecal blood, such as inflammatory bowel disease, ischemic enteritis, vascular alformation of intestine, intestinal tuberculosis or Non-Hodgkin's lymphoma involving digestive tract; People with symptoms including visible rectal bleeding, hematuria, hemorrhoid bleeds, severe and acute diarrhea and watery stool; People are in pregnancy, lactation or menstrual phase; People with severe congestive heart failure or other sever disease cause cannot tolerate colonoscopy.