Peripheral artery cannulation which allows continuous blood pressure monitoring is an easy way for blood sampling. Rarely, peripheral arteries can not be cannulated percutaneously or by cutdown technique. In this article, we describe a six-month-old female infant with a weight of six kilograms whom the left internal mammary artery was cannulated for monitoring arterial blood pressure due to unsuccessful catheterizations of other peripheral arteries. This line continued to function for 10 days postoperatively without any difficulty or complication. The internal mammary artery provides reliable access for blood pressure monitoring and arterial blood sampling in intra- and postoperative period in pediatric patients.