A 5-month-old female toy poodle weighing 1.2 kg was presented for evaluation of a cardiomegaly on thoracic radiographs. Physical examination was unremarkable. Echocardiography revealed continuous shunting flow in pulmonic artery (pressure gradient 118.7 mmHg). The dog was diagnosed as having a left-to-right shunting patent ductus arteriosus. Because no clinical signs, cardiac murmur and hemodynamic changes were recognized, spontaneous closure of ductus arteriosus was expected within a short term. The shunting flow was gradually diminished without left ventricular dilation and right ventricular hypertrophy during follow up. After 2 months, the shunting flow was predictably disappeared. This case was considered a spontaneous closure of patent ductus arteriosus that was expected closing within a short term.