The most challenge for the surgery of low rectal carcinoma was whether to perform low anterior resection (LAR) and preserve anal function improving the quality of life for patients, for which anastomotic leak is a great obstacle with about 5-10% incidence in reported literature. Up to now, kinds of surgical devices have been employed to reduce anastomotic leak rate after LAR. Most of these anastomotic devices could not resolve the problem of "dog ear" phenomena. In the present clinical trial, the investigators use a double purse-string rectal anastomosis with KOL staple in laparoscopic anterior rectal resection for low or ultra-low rectal carcinoma, which will resolve the problem of dog ear. The investigators aim to demonstrate the safety, effectiveness of this procedure and establish a standard method for laparoscopic (ultra-)low anterior rectal resection.

About 5-10% of patients receiving low rectal resection using traditional anastomosis occurred anastomotic leak, risk factors for which included patient related (age, gender, tumor distance from the anal verge), hypoproteinemia, diabetes mellitus, etc), procedure related (emergency) and technical related (one stapler, double stapler or handsewn).

Anastomotic leak caused prolonged hospital stay, delayed post-operative adjuvant radiochemotherapy and need of stoma in some cases. Besides, anastomotic leak was reported to be associated with increased local recurrence rate. Thus, it's of great importance to develop new surgical devices to prevent "dog ear" problem and reduce the anastomotic leak rate.

Double stapled pursestring anastomosis was the one of the most widely used methods in lower rectal resection. One major problem of this procedure was creating one or double side "dog ear" phenomenon (Dis Colon Rectum. 2000 Apr;43(4):522-5. Fig 1). This weak spot was theoretically responsible for post-operative anastomotic leak as demonstrated by animal experiments and clinical practice.

In this clinical trial, we used a transanal double purse-string rectal anastomosis preformed with KOL stapler for lower rectal resection. This procedure would resolve the dog ear problem through circular anastomosis taking care that the anastomotic site contained only gut tissues without any staples. We aim to demonstrate the safety, effectiveness of this procedure and establish a standard method for laparoscopic (ultra-)low anterior rectal resection.

Inclusion Criteria:

1. pathological confirmed rectal adenocarcinoma
2. less than 10 cm of distal tumor margin from the anal edge
3. less than 4 cm of maximum tumor diameter and less than 1/2 of circumference diameter
4. tumor stage earlier than cT1-3N0M0 before surgery or that following neoadjuvant radiochemotherapy
5. normal defecation function (Wexner score < 4)
6. open or laparoscopic operation

Exclusion Criteria:

1. recurrent cases
2. emergency including obstruction, bleeding or perforation
3. severe abdominal adhesions
4. severe malnutrition can not be improved before surgery
5. can not tolerate to surgery due to severe comorbidities of heart, lung, liver or kidney
6. refractory hypoproteinemia or diabetes mellitus