In this randomized clinical trial, two groups of preterm infants (each group contains 44 infants, gestational age 28-37 weeks) with respiratory distress syndrome (RDS) in NICU of Al-Zahra or Shahid-Beheshti hospitals who are on the Nasal Continuous Positive Airway Pressure (NCPAP) with P=5 CmHg and Fio2≤30% at least for 6 hours, in stable state (respiratory rate<60, without pulmonary resuscitation, having no apnea and bagging, having less than 6 apnea episodes) will be studied. Written informed consent will be obtained from parents. Then we randomly allocate infants into two groups; HHFNC group (Humidified High Flow Nasal Cannula) who dissent from NCPAP at Fio2≤30% and receive 2 Lit/min O2 via cannula nasal, control group who are continued to the NCPAP till Fio2=21% to achieve a stable condition for 24 hours. In HHFNC group, O2 is adjusted according to infant need to maintain the O2 arterial pressure between 87 and 92%. The O2 flow is not changed till Fio2=21%, then its flow will decrease each 6 hours to achieve 0.5 L/min and at this time infant will be disparted. To maximize the efficacy, prong size will be appropriate to infant’s nose. Successful weaning is considered when infant could be stable for 72 hours in the room temperature. Statistical analysis will be conducted using SPSS version 20 , independent t-test, and chi-square test will be used to test significant differences between groups.