Background. Two types of end-to-end colorectal anastomosis using a circular stapler, circular stapled anastomosis(CSA) and double stapling anastomosis(DSA), have become the standard technique. The possibility of anastomotic openings of the DSA being smaller and somewhat distorted, because of some portion of the stapled rectal stump not being included in the anastomotic opening, was examined.Methods. Anastomotic openings created by CSA and DSA were photographed through a sigmoidoscope, and the maximal and minimal diameters and the areas of the anastomotic openings were measured and calculated with an image analyzer. They were examined in the swine rectum immediately after surgery and were also examined in the randomized clinical cases 3 or more months after surgery.Results. It was found that larger anastomotic openings could be created as the diameter of the anastomosed intestine and the cartridge size became wider in both methods immediately after surgery. The shape of anastomotic opening created by DSA was observed to be more oval than round immediately after surgery. The anastomotic stenosis occurred in a small number of patients in both methods within 3 months after surgery. The area of the anastomotic opening created by CSA was about 25% larger than that by DSA immediately after surgery and about 30% larger 3 or more months after surgery.Conclusion. CSA provides a larger anastomotic opening than DSA. Anastomotic stenosis occurred in a small number of patients in both methods.