Left atrial appendage aneurysm (LAAA) is a rare cardiac anomaly. The cause mostly due to congenital, but can be acquired also. Patient may remain asymptomatic or may present with variable symptom. It can predispose to hazardous adverse events, including atrial fibrillation, myocardial infarction, cardiac dysfunction and life-threatening systemic thromboembolism. Simple imaging, electrocardiography and echocardiography can diagnose this rare cardiac anomaly. We are reporting a case who presented to us at 5 years of age with palpitation, chest pain and dizziness with arrythmia that developed one month back; he visited our outpatient department of the National Heart Foundation Hospital & Research Institute Hospital, Dhaka, Bangladesh on 13th February 2020. We diagnosed left atrial appendage aneurysm with mitral valve prolapse with atrial arrhythmia thereafter surgical resection of aneurysmal part along with mitral valve annuloplasty done by mid sternotomy and maze therapy. Postoperative period was uneventful and discharged after 6th post operative day.