Background Acute respiratory failure develops in infants with bronchiolitis and Community acquired pneumonia (CAP) because of hypoxemia, due to mismatch between ventilation and perfusion. Nasal continuous airway pressure (nCPAP) and high-flow nasal cannula (HFNC) improve the work of breathing and oxygenation .High flow nasal cannula (HFNC), also called heated humidified high flow nasal cannula (HHHFNC), is a new non-invasive ventilation therapy that seems to be well tolerated in infants with hypoxemic respiratory failure. Aim To compare the effectiveness and the outcome of Heated Humidified High Flow Nasal Cannula (HFNC) versus Nasal Continuous Positive Airway Pressure (NCPAP), as a primary mode of respiratory support, in patients with respiratory failure due to pneumonia or bronchiolitis at PICU regarding demographic data, length of admission, increasing of positive pressure and need of intubation. Patients and Methods observational study was conducted on 40 patients with acute respiratory failure due to either bronchiolitis or community acquired pneumonia (CAP), admitted to Paediatric ICU, Children Hospital, Ain Shams University, in the time period between February 2018 to July 2019. The patients, aged between 1 month to 5 years, were subjected to detailed clinical history and examination.All patients received continuous monitoring of electrocardiograph ,pulse oximetry and respiratory rate .The patients were treated either with (HFNC) Humidified high flow nasal cannula or continuous positive airway pressure (CPAP). Studied groups were compared regarding demographic data, increasing in respiratory support ,need of intubation, fate and length of admission during PICU stay. Results HFNC was as efficient as CPAP in lowering RR(respiratory rate) and HR(heart rate) in infants with bronchiolitis and community acquired pneumonia. Thus HFNC group was as significant as NCPAP group in treatment of bronchiolitis and CAP regarding length of admission ,increasing of positive pressure and need of iintubation in PICU . Conclusion This study concluded that HFNC is as efficient as nCPAP for initial respiratory support in young infants, hospitalized in a PICU for moderate to severe acute respiratory failure.