Background: For infants receiving general anesthesia (GA), it is crucial to select the proper endotracheal tube (ETT) size since an unsuitable tube might damage the airway and cause subglottic stenosis. On the other hand, a smaller tracheal tube increases the resistance to gas flow, the danger of aspiration, the requirement for inadequate ventilation, and the necessity of reintubation with a different size tracheal tube. Objective: The aim of the present study was to investigate the efficacy of the minimum transverse diameter of the subglottic airway (MTDSA) as a reliable method for selection of the tubal size in infants. Patients and methods:  A randomized controlled clinical trial was conducted on infants aged >1 year underwent surgery at Souad Kafafi University Hospital-Misr University of science and Technology. Infants were divided in two equal groups; Group A underwent measurement of MTDSA (21 infant), and Group B used the conventional method (internal diameter (ID) of the most frequently used ETT were 3.0 mm for infants less than 1 month old, 3.5 mm for those 1 to 4 months old, 4.0 mm for those 5 to 17 months old) (21 infants). Results: The use of MTDSA as a method for selection of the tubal size could predict up to 86.7% of correct tube size in contrast to only 47.6% in group B; there was a highly significant difference between the 2 studied groups regarding the internal and external diameters of the selected tubes. Conclusion: Compared to other approaches, MTDSA using ultrasonography has a higher sensitivity for predicting the size of an ETT. Younger infants have more ultrasonographic sensitivity than older ones.