Circular myotomy is useful method for bridging the gap in the case of esophageal atresia with wide gap. But its indication is still unclear because the maximal permissable tention for safe anastomosis and precise effects of circular myotomy on length and tention at anastomosis are not known. This study was undertaken to answer these questions. In 18 piglets that thoracic esophagus was excised from Ocm to 6.5cm, approximtion force between both ends was measured by dynamometer prior to anastomosis. When the approximation force was greater than 75 grams, anastomotic leak and marked scar formation was likely to occur at anastomosis. In piglets and neonate cadavers study, the upper third of esophagus was stretched by application of the traction force of 0-150 grams. At each traction force, the length of upper esophagus was measured before and after circular myotomy. Circular myotomy increased esophageal length by about 5mm for any traction force. To know the actual approximation force at the time of esophageal atresia repair is very helpful to make a decision for circular myotomy. If greater than 75 grams of force is necessary to approximate each ends, circular myotomy is indicated.