We report a case of acute thrombosis of bioprosthetic mitral valve in a 59 year-old Korean female, who underwent a mitral valve replacement with a 25 mm Carpentier - Edwards PERIMOUNT Plus bioprosthesis (Edwards Lifesciences, Inc.; Irvine, CA, USA) and a mini-Maze procedure for correction of mitral stenosis (MS) and atrial fibrillation (AF). On the 10th postoperative day, the patient began to complain of increasing dyspnea and general malaise. Her symptoms worsened and developed into pulmonary edema. Echocardiography revealed a mean diastolic pressure gradient (MDPG) of 10 mmHg across the mitral valve and pressure-half time (PHT) of 166 msec. Due to progressive decompensated heart failure, the patient underwent a repeat sternotomy to replace the bioprosthetic mitral valve. Intraoperatively, we found a thrombosis around the bioprosthetic mitral valve. We excised the bioprosthetic mitral valve and replaced it with a 27 mm ATS mechanical valve (ATS medical, Inc.; Minneapolis, MN, USA). We experienced a rare case that required an early reoperation for a thrombosis of the bioprosthetic valve.