One case of primary omental torsion in a 10 year-old, 43 kg boy is presented. He presented with a history of acute, continuous pain in the upper abdomen, aggravated by changes of position. Physical examination of his abdomen showed board-like rigidity. tenderness and rebound tenderness over the entire abdomen. The preoperative diagnosis was perforated peptic ulcer. Exploratory laparatomy revealed torsion of the greater omentum. The torsed omentum was excised and the outcome was good. Omental torsion is a rare surgical condition and is difficult to diagnose prior to operation. Therefore, in case of negative exploration for acute abdomen, the omentum should be included among the organs examined. In addition, careful search for an inflammatory focus should be made, because secondary omental torsion with intraabdominal sepsis is much more common than primary torsion.