Background: It has been controversial whether the mitral annulus contract or dilate during systole. This study was done to explore the change of shape and area (2D and 3D) of the annulus during systole using real-time 3D echocardiography (RT3DE) and newly developed 3D computer software (TomTec). Methods: Thirteen normal subjects with normal structure of the mitral apparatus underwent RT3DE. 3D shape of the mitral annulus was reconstructed through 3D coordinates of annulus traced on 16 rotational apical planes during early, mid and late systole. 3D surface (3DMAA) and 2D projected mitral annular area (2DMAA) was automatically calculated. Non-planarity of annulus was estimated by non-planar angle (NPA) between 2 vectors from two hinge-points of the annulus in the AP plane to the center of the axis connecting two commissures in the CC plane. Results: NPA significantly increased from early to late systole (146짹6 vs 151짹5 vs 156짹6쨘, p< 0.01, respectively). There was significant increase of both 3DMAA and 2DMAA both from early to mid systole (3DMAA: 9.5짹1.3 vs 9.9짹1.3 cm2, p = 0.02, 2DMAA: 8.5짹1.3 vs 9.2짹1.4 cm2, p = 0.01) and mid to late systole (3DMAA: 9.9짹1.3 vs 10.8짹1.4 cm2, p < 0.01, 2DMAA: 9.2짹1.4 vs 10.1짹1.4 cm2, p < 0.01). Conclusion: The mitral annulus enlarged during systole with lessening its non-planarity. The geometric change of the mitral annulus seems to be more closely associated with the left atrium than the left ventricle during systole.