Background: Pediatric patients with congenital heart diseases may have pathological airway abnormality and delayed development. To predict the appropriate size of endotracheal tube (ETT), a formula between diameter and age has been widely used for Western normal children. However, it is unclear whether this age-based (AB) formula is applicable to Thai pediatric cardiac patients. Objective: Evaluate the effectiveness of uncuffed ETT size by AB formula for pediatric cardiac patients. Methods: A retrospective study was conducted using 320 cases of non-cardiac and cardiac patients aged 2-7 years old who were orally intubated with a regular uncuffed ETT at Siriraj Hospital, Thailand. The exclusion criteria were history of tracheostomy, upper airway obstruction, and expected difficult intubation. Demographic data and final ETT used were recorded. Results: The tube- size predicted by the AB formula could be applied to 54.4% of non-cardiac and 48.1% of cardiac patients (p= 0.314), whereas three sizes of tubes (one above and one below the predicted size) covered 96.9% and 94.4% of non-cardiac and cardiac patients, respectively (p = 0.413). The ETT with 0.5 mm in ID larger than the predicted size were more often used in 35.0% of cardiac patients compared with 22.5% of non-cardiac patients (p= 0.019). There were no significant differences between methods using age (actual, round-up, and truncated) to calculate the AB formula. The Pearson’s correlation between the ID of the ETT with height in non-cardiac and cardiac patients were 0.430 and 0.683, respectively (p <0.001), whereas correlations with weight were 0.622 and 0.561 (p <0.001), respectively. Conclusion: The AB formula was applicable to non-cardiac and cardiac children aged 2-7 years old. For Thai pediatric cardiac patients, we recommend to use a one-size larger ETT than non-cardiac patients.