There have been various arguments concerning the patient-prosthesis mismatch (PPM) after aortic valve replacement (AVR) for small valves. The objective of this study was to evaluate the postoperative hemodynamics in patients. Methods: The subjects were 6 patients [6 females aged 64 (17~74) years, with a median body surface area (BSA) of 1.37 (1.29~1.51) m²] who underwent AVR at our facility using the 18-mm ATS-AP and tolerated exercise loading. We estimated pressure gradient(PG) ,ejection fraction(EF), left ventricular mass Index(LVMI)at pre-operation and post-operation. Exercise echocardiography on an ergometer was performed before and 29.0 ± 14.4 months after surgery. We estimated PG and effective orifice area(EOAI) at rest and at exercise. We compared echo data between pre-operation and post-operation, between at rest and at exercise. Results: The effective orifice area index(EOAI) at rest was 0.92(0.75~1.06) cm²/m². There was a significant change in the LVMI between pre-operation and post-operation[158.5(104.0~222.2) g/m² versus 102.4(92.3~146.4) g/m²; P < 0.05]. However, There was no significant change in the EOAI[0.92(0.75~1.06) cm²/m² versus 0.84(0.78~0.91) cm²/m²; P > 0.05] and mean PG[11.0(6.6~16) mmHg versus 14.0(6.3~16) mmHg; P > 0.05], on maximal exercise. Conclusion: In patients whose BSA were 1.37(1.29~1.51) m²,the 18-mm ATS-AP was suggested to be a prosthetic valve that improves myocardial remodeling and provides stable hemodynamics even during exercise.