Heart rhythm disorder is a frequent concomitant disease of cardiovascular and extra-cardiac pathology. A rare variant of heart rhythm disorder, which is not typical for newborns and young people, is atrial fibrillation. Frequency of atrial fibrillation is less than 1 %. The main purpose is analysis of clinical case in newborn with atrial fibrillation. Diagnostic and search effective methods of treatment atrial fibrillation in newborn’s is an actual problem in child cardiology. In this article described successful case of treatment life-threatening atrial fibrillation in newborn by electric cardioversion without cardiovascular pathology. Electric cardioversion in newborn’s with supraventricular tachycardia, which have hemodynamic disorders, is safe and effective therapy method.