Background: Choosing the correct endotracheal tube (ETT) size is important in pediatrics patients as an inappropriately large sized tube may cause damage to the airway and subglottic stenosis. On the other hand, a smaller tube will increase the resistance to gas flow, risk of aspiration, insufficient ventilation, and the need to re-intubate with a different size of tracheal tube. Objective: To compare the accuracy of measurement of subglottic transverse tracheal diameter using ultrasonography and aged based formula for prediction of uncuffed endotracheal tube size in children aged 2-12 years submitted to general anesthesia. Patients and Methods: After approval of scientific and ethical committees, 88 children aged 2- 12 years submitted to general anesthesias in Al-Azhar University Hospitals were enrolled in this study from May 2019 to May 2021. Children were allocated in two groups: Group I where subglottic transvers tracheal diameter, outer diameter of ETT, inner diameter of ETT, inspiratory tidal volume, expiratory tidal volume, leak volume and leak % were measured and calculated after induction of general anesthesia and Group II where inner diameter of uncuffed ETT was calculated according to cole formula (0.25 (age in years) + 4. Results: Ultrasonography (USG) was more accurate in prediction of uncuffed ETT size with sensitivity 86.4%, while the sensitivity of the age based formula was 68.2%. Conclusion: The sensitivity of ultrasonography in prediction of ETT size was superior to age based formula Ultrasonography was more sensitive in yonger children than in older children.