Background. Cor triatriatum is a rare congenital cardiac anomaly, represent 0.1% of all congenital cardiac malformations and may be associated with other cardiac diseases in as many as 50% of cases. The natural history of this defect depends on the size of the communicating orifice between the upper and lower atrial chamber. Case Presentation. We reported case of cor triatriatum in a 12 years old girl with chief complaint of shortness of breath, middle chest discomfort and palpitation since 5 days prior admission. The diagnosis was based on clinical features, chest radiography, electrocardiography and transthoracic echocardiography. Chest radiograph showed rounded cardiac apex and double contour appearance. ECG showed sinus rhythm, 75 beat per minute, RAD, CCWR, RVH, RV strain pattern with ST depression and T-wave inversion in II, III, aVF, V1-V5. TTE revealed 2 chambers of left atrium, with restrictive supramitral membrane, dilated right atrium, right ventricle and left atrium, smallish left ventricle, proximal left atrial thrombus (5.96 x 3.44 cm), relative mitral stenosis, severe mitral regurgitation, mild aortic regurgitation and severe tricuspid regurgitation. A diagnosis of cor triatriatum sinister was made. The only treatment is surgical correction. Medical therapy, with ampicillin and heparin, was administered during admission. Conclusion. Cor triatriatum has been reported in a 12-year-old girl. The diagnosis is confirmed by clinical manifestations, chest radiography and echocardiography. The only therapy is surgical correction. From the field of cardiac surgeon, patients are advised to improve their general conditions before underwent surgical procedures.