Abstract: Objective：To study the application value of realtime threedimensional transesophageal echocardiography (RT3DTEE) in measuring the maximal atrial septal defect (ASD) diameter. Methods：The clinical data of patients diagnosed as central, secundumtype, singlehole ASD and hospitalized for transcatheter closure from January 2011 to June 2014 in a single center were prospectively analyzed. The maximal ASD diameters measured by RT3DTEE, twodimensional transthoracic echocardiography (2DTTE) and twodimensional transesophageal echocardiography (2DTEE) were compared. Correlation between the maximal ASD diameters measured by three methods and occluder waist diameter after release were analyzed. Results：A total of 31 ASD patients were included. There was no significant difference between the maximal ASD diameters measured by RT3DTEE (［21.81±8.57］ mm), 2DTEE (［20.31±8.50］ mm), and 2DTTE (［23.08±10.89］ mm) and occluder waist diameter after release (［20.99±8.15］ mm). The maximal ASD diameters measured by RT3DTEE, 2DTEE, and 2DTTE were all highly correlated with the occluder waist diameter (r=0.902, 0.981, 0.897, respectively, P=0.000). Conclusions：Compared with twodimensional echocardiography, RT3DTEE has no advantage in measuring the maximal ASD diameter.