Pericardial effusions and cardiac tamponade are rare and severe complications of leukemia. They often develop during the radiation therapy, chemotherapy, or infections in the course of leukemia. However, some cases present with pericardial effusion and tamponade. We report a three-year-old girl who was admitted with cardiac tamponade and needed urgent pericardiocentesis. Clinical evaluation and laboratory results revealed myeloid markered-T cell acute lymphoblastic leukemia (ALL) and pericardial invasion. She is the youngest patient with cardiac tamponade who was diagnosed acute lymphoblastic leukemia in the English-language literature.