The Roux limb is the most commonly used procedure for hepato-biliary surgery in pediatric cases. Notwithstanding, the postoperative status of the limb has not been well described. In this report, our group attempted to observe the Roux limb ultrasonically after performing a Roux-en-Y jejunostomy. Hepatic porto-jejunostomy and hepatico-jejunostomy were performed in 2 patients with biliary atresia and 4 patients with congenital bile duct dilatation, respectively. The Roux limb was observed by real-time ultrasonography from postoperative day (POD) 1 to POD 12. The wall thickness and luminal diameter of the Roux limb were measured, and the movement of the limb was observed. In the 2 hepatic porto-jejunostomy patients, the wall thickness was increased on POD 1 but thereafter improved from PODs 3 to 6. The lumen of the Roux limb was difficult to describe in one of the patients because of an inadequate amount of bile flow. Cooperative peristalsis appeared on PODs 4 to 6 following the commencement of irregular movement from PODs 2 to 3. In the 4 hepatico-jejunostomy patients, the wall thickness was increased by POD 1 but improved from PODs 2 to 6. Cooperative peristalsis appeared on PODs 4 to 5 following the commencement of irregular movement. In conclusion, ultrasound examination is useful for observing the postoperative morphological aspects of the Roux limb.