Objectives: High-flow nasal cannula (HFNC) has arisen as a novel treatment method for providing high-flow oxygen support. It can be used for patients of all age groups, provides respiratory support in respiratory tract diseases, and its use is rapidly increasing. The aim of the study was to compare the effectiveness of oxygen therapies with HFNC and a non-rebreathing face mask (NFM) with a reservoir bag through changes in vital signs before and after treatment. Methods: Patients aged under two years who were diagnosed as having acute bronchiolitis were included in study. Of the randomly selected patients, one-half was given HFNC oxygen therapy and the other half was given standard oxygen support via an NFM. Results: There was a significant reduction in respiration rates (RR) at the 3rd hour and in heart rate (HR) at the 6th hour of treatment compared with NFM. Time to normalization of HR and RR according to age and length of hospital stay were shorter and need for intensive care support was less in those receiving HFNC oxygen support. Conclusions: HFNC significantly shortens length of hospital stay and duration of oxygen therapy compared with standard oxygen. The authors believe that the effectiveness of treatment or response to treatment could be evaluated using HR and RR monitoring. A flow rate up to 25 L/min could be used for patients aged under two years.