Cardiac pseudoaneurysm is an extremely rare condition in children. It can be congenital or acquired and has been primarily described in the left side of the heart. The congenital form of pseudoaneurysm is thought to be related to muscular dysplasia of the muscular layer of the heart. The acquired form is related to myocardial infarction, infective endocarditis or chest trauma. In this report we described a 5-month-old baby with left atrial pseudoaneurysm who was referred to our hospital due to tachycardia and hemodynamic instability. After diagnosis of left atrial pseudoaneurysm, aneurysmectomy was performed and the left atrial wall was successfully repaired by surgery three days after admission. In conclusion, due to life threatening complications such as dissection, arrhythmias, compression of aneurysm and tromboembolic complications, pseudoaneurysm should be treated by surgery.