Background: Coronary sinus dimension is an important factor for diagnosing some types of cyanosis as well as congenital heart diseases and insertion of some devices into the coronary sinus if required. This study was designed to access the diameter and Z-score of the coronary sinus among children under 18 years of age. Methods: In this cross-sectional study on 95 individuals, the coronary sinus diameter was measured by transthoracic echocardiography in the four-chamber view at the connection to the right atrium, middle part, and distal end. The linear regression equation was utilized to determine age-adjust reference values, Z-scores, and the relationship between the individuals’ coronary sinus diameter age, height, and body surface area. The study was conducted from March to July 2020 in Namazi hospital clinic of Shiraz University of Medical Sciences, Shiraz, Iran. Results: The mean age of the persons who entered this study was 5.87±4.25 years. The mean coronary sinus diameter was 4.91±1.29 mm at the site of connection to the right atrium, 4.50±1.44 mm at the middle part, and 3.74±1.32 mm at the distal end. Coronary sinus diameter correlates positively with the participants’ age, weight, height, and body surface area (P<0.001). Conclusion: Coronary sinus diameter significantly correlates with the age, height, and body surface area of the cases. These features are useful in diagnosing some congenital heart diseases and insertion of suitable devices through it.