To investigate the mechanism and correlated factors of systolic anterior motion (SAM) phenomenon after aortic valve replacement, 48 patients with severe aortic valvular stenosis were studied. Tested by echo-Doppler one week after aortic valve replacement, the patients were divided into two groups: SAM group and non-SAM group. The data of the left ventricular end-diastolic diameters, the left ventricular end-systolic diameters, the left ventricular outflow diameters, the thickness of the interventricular septum, the posterior wall of left ventricle, the blood velocities of left ventricular outflow and intra-cavitary gradients were recorded and compared. The results showed that no patients died during or after the operation. The blood velocities of left ventricular outflow was increased significantly in 9 patients (＞2.5 m/s), and 6 of them developed SAM phenomenon. There was significant difference in all indexes (P＜0.05 or P＜0.01) except the posterior wall of left ventricle (P＞0.05) between two groups. These indicated that the present of SAM phenomenon after aortic valve replacement may be directly related to the increase of blood velocities of left ventricular outflow and intra-cavitary gradients. It is also suggested that smaller left ventricular diastolic diameters, left ventricular systolic diameters, left ventricular outflow diameters and hypertrophy of interventricular septum may be the anatomy basis of SAM phenomenon.