Abstract: Objective To compare the results of 2D and 3D echocardiography in measuring aortic annulus diameter (AAD). Methods Preoperitve transesophageal echocardiography (TEE), transthoracic echocardiography (TTE) and 3D transesophageal echocardiography (RT 3D TEE)were performed on 52 patients who underwent aortic valve replacements to measure AAD. The parameters were compared between systole and diastole. Results The parameters in systole were significant greater than those in diastole (P<0.05). No significant differences in the AAD value were found between TTE and TEE. However, the AAD values determined with TTE and TEE were different from those determined with RT 3D TEE (P<0.05). The two-dimensional measurements produced smaller AAD values in both systole and diastole than the 3D measurements (P< 0.001). TEE correlated well with RT 3D TEE, with the r value of 0.775 and 0.765 for systole and diastole, respectively. The regression was 3D-AADs=0.531×TEE-AADs+19.879 (mm). Conclusion RT 3D TEE can accurately reconstruct the 3D structure of aortic annulus and calculate the aortic annulus dimension. TEE-AAD correlates well with 3D-AAD.