Uncuffed endotracheal tubes are commonly used in children in an attempt to decrease the potential for pressure induced tracheal injury. However, uncuffed endotracheal tube may increase the risk of aspiration and lead to erratic delivery of preset tidal volume during mechanical ventilation. Therefore, it is desirable to intubate trachea with an appropriate but not an oversized endotracheal tube. In children, for selecting an endotracheal tube, a variety of formulas and techniques are used to find the endotracheal tube size that minimizes both pressure induced tracheal injury and aspiration potential or variable ventilation. Air-leak following tracheal intubation can be recognized by the presence of audible leak, by auscultation over the trachea, by palpation over the trachea and by observing effects of positive end-expiratory pressure on inspiratory expiratory tidal volume difference during mechanical ventilation. We describe mainstream time-capnograph as an aid to recognize leak around the endotracheal tube and its utility to determine appropriate endotracheal tube size in small children.