Although the pressure gradient (PG) and the effective orifice area (EOA) have been used as indices of prosthetic valve function, these values show correctly neither energy loss, nor increased workload. This study aimed to evaluate the prosthetic valve function using echocardiography and PG, EOA and energy loss index, a new index advocated by Garcia et al. These were calculated for 40 patients with aortic prosthetic valve replacement by SJM valve (19HP, 6 cases; 21mm, 16 cases; 23mm, 14 cases; 25mm, 4 cases). Preoperative and postoperative echocardiographic measurements and their variations were analyzed and compared according to the size of implanted valve. In the comparison before and after aortic valve replacement, left ventricular mass (383±151g vs 288±113g, p<0.01), SV1+RV5 on ECG (5.07±1.73mV vs 3.83±1.5mV, p<0.01), and diastolic left ventricular posterior wall thickness (14.4±3.7mm vs 12.9±2.8mm, p<0.05) decreased significantly after the operation. However, there was no significant difference according to the size of the prosthetic valve in these reduction rates caluculated by (preoperative value-postoperative value)/preoperative value. Small size prosthetic valves were used for patients with small diameter of left ventricular outflow tract (LVOT) (19HP, 18±2mm; 21mm, 21±2mm; 23mm, 23±4mm; 25mm, 27±3mm; p<0.01) and small body surface area (19HP, 1.5±0.2m²; 21mm, 1.5±0.2m²; 23mm, 1.7±0.1m²; 25mm, 1.8±0.1m²; p<0.01) in our study. There was a signifcant difference in EOA (19HP, 1.2±0.4cm²; 21mm, 1.9±0.7cm²; 23mm, 2.2±0.9cm²; 25mm, 3.5±1.1cm²; p<0.01), but not in ELI (19HP, 1.01±0.41cm²/m²; 21mm, 1.87±1.03cm²/m²; 23mm, 1.83±1.09cm²/m²; 25mm, 3.08±1.21cm²/m²; p=0.055) according to the size of the prosthetic valve. Small size prosthetic valves had small EOA, but showed satisfactory valve function in decreasing left ventricular hypertrophy and reducing LVM and ELI of small size was similar to that of large size.