Valvular heart disease is highly prevalent, especially in developing countries. Mitral Stenosis (MS) is a condition where there is narrowing of mitral heart valve. Left atrial (LA) thrombus is often seen in severe MS patients. A 47-year-old woman complained of palpitation and shortness of breath. The heart sounded irregularly irregular, with grade III/IV diastolic murmurs at the apex. Her electrocardiogram showed atrial fibrillation (AF) with rapid ventricular response Transthoracal echocardiography (TTE) showed severe MS, mild tricuspid regurgitation, and LA thrombus. Mitral valve replacement surgery, tricuspid valve repair, and evacuation of the LA thrombus were immediately done. We evacuated a spherical mass with a size of 4 × 3x2.2 cm, layered and easily separated. Microscopic examination showed extensive fibrin and bleeding with mononuclear inflammatory cells and macrophages, corresponding to a thrombus conclusion. Atrial thrombus is common in MS patients. The incidence will increase by about two times in patients with AF. TTE is a reliable tool in diagnosing large mobile atrial thrombus and differentiated it from other cardiac masses. However, histopathological examination is still the gold standard to distinguish between LA thrombus and myxoma. Immediate thrombus evacuation and valve replacement, if needed, will give good results and reduce systemic thromboembolism. LA thrombus is often seen in a patient with severe MS. Optimal preoperative preparation involves assessing preoperative risk stratification will give good results.