Background: 3D shape and geometry of the tricuspid annulus has not been clarified yet. The aim of this study was to explore 3D shape and size of normal tricuspid annulus comparing with the mitral annulus using real-time 3D echocardiography (RT3DE) and newly developed 3D computer software. Methods: Thirteen Left ventricular (LV) and thirteen right ventricular (RV) volumetric images were acquired using RT3DE from 22 normal subjects. LV and RV volumetric data were segmented into 16 rotational apical planes (angle increment = 18/16째) around the rotational axis from the apex through the center of the mitral and tricuspid annulus, using newly developed 3D software (TomTec, Co). Two hinge points of leaflets were traced in each plane during early and late systole. The mitral and tricuspid annuluses were then reconstructed with those 32 traced points by fitting and interpolation. 3D surface and projected area of the mitral and tricuspid annulus were automatically calculated from those fitted data. Results: 3D shape of the tricuspid annulus showed round or oval shape with flatter appearance compared to the mitral annulus revealing "saddle" shape appearance (Figure). Both 3D surface (10.8짹2.1 vs 9.5짹1.3 cm짼, p = 0.02) and projected area (10.1짹2.2 vs 8.5짹1.3cm짼, p = 0.01) of the tricuspid annulus were slightly larger than those of the mitral annulus during early systole. However, there was no significant difference (p > 0.05) in both 3D (11.1짹2.3 vs 10.8짹1.4 cm짼, p = 0.4) and 2D area (10.5짹2.4 vs 10.1짹1.4 cm짼, p = 0.4) between two annuluses during late systole Conclusion: We assessed 3D geometry of the mitral and tricuspid annuluses after 3D reconstruction of the annulus with newly developed 3D computer software. Tricuspid annulus revealed more round and planar shape compared to the mitral annulus with less geometrical change during systole.