The purpose of this study is to determine whether the supine position is better than the left lateral horizontal body position to decrease cecal intubation time, patients' pain and the difficulty of colonoscopy insertion.

Left lateral position is conventionally performed to initiate colonoscopy, yet no evidence has demonstrated its efficacy and advantages. Left lateral position for insertion, which results in air rising away from the left colon and producing acute bends in the sigmoid,might increase the risk of colonoscopy loop formation.When there is difficulty advancing the colonoscope, supine position can be employed to ensure cecal intubation. Therefore, we perform a randomized controlled trial to compares supine position with left lateral position for the safety and efficacy of colonoscopy insertion.

Inclusion Criteria:

1. aged between 18-75 years
2. underwent screening, surveillance, and diagnostic colonoscopy

Exclusion Criteria:

1. <18 or >75 years old
2. major psychiatric disorders, colonic resection, pregnancy, presence of any contraindications for colonoscopy (eg, severe heart failure, renal insufficiency)
3. Patients with severe cardiopulmonary and renal disease
4. Patients who are unwilling or unable to consent