Acute respiratory failure is characterized by an acute onset of hypoxemia with a high morbidiity and mortality rate in critically ill patients. Although mechanical ventilation is still the mainstay therapy for severe hypoxiemia, with the technical progress in high-flow nasal cannula (HFNC), the latter has been widely applied to the acute respiratory failure patients because of its physiological effects: reduction of anatomical dead space, delivering a positive end expiratory pressure (PEEP) effects, increasing end-expiratory lung volume and a better comfortness. However, it still needs to be concerned about the indication, the exact PEEP effect, HFNC associated lung injury and the timing of mechanical ventilation when treating acute respiratory falure patients with HFNC.