Ｏｂｊｅｃｔｉｖｅ　To evaluate the clinical effect of measuring pediatric subglottic diameter under ultrasound for uncuffed tracheal tube size selection in a pediatric population.　Methods　A total 45 pediatric patients who scheduled for surgery and undergoing tracheal intubation were enrolled. In this study, researchers determined tracheal tube sizes by ultrasound according to subglottic diameter. The final determine standard of tube size is whether a good clinical fit reached. In the meantime, calculating a tube size according to the age-based formula. We compared the accuracy of measuring the subglottic diameter under ultrasound with the accuracy of determining tracheal tube size by age-based formula. We also did the correlation analysis between different factors, which included the age, height, weight and the subglottic diameter, and the optimal tube size(ID) separately.　Results　The accuracy of assessing subglottic diameter under ultrasound，which is 89%, is better than the age-based formula, which is 51%. The difference was statistically significant(P＜0.05). The subglottic diameter has a strong correlation with ID than other factors.　Conclusions　Predicting the uncuffed tracheal tube size according to subglottic diameter, determined by ultrasound, is a new simple way in clinic.