One lung anaesthesia in paediatric patients may not always be achievable by bronchial blockade or double lumen tube intubation due to inadequate experiences or facilities. We attempted to isolate right lung by selectively intubating the left bronchus with single lumen tube on a 10 kg child. Optimal surgical condition and satisfactory oxygenation achieved but complicated with severe respiratory acidosis. The possible causes for hypercapnea in this child were discussed.