Perfusion Index (PI) which reflects the peripheral blood flow may help early detection and treatment decision of hemodynamically significant patent ductus arteriosus (hsPDA) in preterm infants. The present study is designed to analyze the usefulness of PI level in early detection of hsPDA in preterm infants.Preterm infants born before 36 gestational weeks were assessed for PI and simultaneous echocardiography. Based on echocardiography, each infant is categorized into no-PDA (group 1), non-hsPDA (group 2) and hsPDA (group 3). Heart rate (HR), mean arterial pressure (MAP), body temperature and oxygen saturation (SpO2) and concomitant PI were measured on days 1, 2, 3 and 4.In all preterm infants (N.=42) PI significantly increased from 0.7 on day 1 to 1.4 on day 4. The HR did not change by the days; however, the MAP increased on days 3 and 4 compared to day 1. In hsPDA group, the median PI was 0.7 (IQR, 0.4) on day 1 compared to 0.9 (IQR, 0.2) on day 2. PI is significantly lower in hsPDA group compared to no-PDA group on day 1 and 2; however, this difference disappeared at 48 hour on the intravenous ibuprofen treatment (on day 3 and 4).PI may predict the perfusion disorder and help to decide for treatment of hsPDA and was also helpful to monitor the response to treatment in hsPDA patients.