One-lung ventilation is very rarely associated with tension pneumothorax which can progress rapidly to become a life threatening situation when it develop in the ventilated dependent lung during one-lung ventilation. A 59-year-old female patient with bronchiectasis underwent double-lumen endotracheal tube intubation for left lower lobectomy. Our patient's presenting signs were decreased tidal volume, increased airway pressure, decreased oxygen saturation and bronchoscopic finding of collapsed bronchus during one-lung ventilation. In our case, hypotension was transiently noticed but recovered without any treatment. Resumption of two-lung ventilation did not relieved these signs. A diagnosis of tension pneumothorax in the ventilated dependent lung was confirmed by chest X-ray.