Introduction: The first line treatment of acute respiratory failure is supplemental oxygen administration. The oxygen provide by conventional oxygen delivery system may not suffice in patients with acute respiratory failure. Recently, an alternative to conventional oxygen therapy has received growing attention: heated, humidified high flow nasal cannula (HFNC). However, precise indication of HFNC and outcome of patients undergoing HFNC are unknown. Objective: The purpose of this study was to determine the effectiveness of high flow nasal cannula (HFNC), and the factors predicting from failure. Method: This was a prospective observational study. Patient with acute respiratory failure were treated with HFNC oxygen therapy at Chuncheon Sacred Hospital, between December 2011 and April 2012. Clinical respiratory parameters and arterial blood gases were compared under HFNC oxygen therapy. Result: In all, 33 patients were included in the study. Their median age was 71.1 years (29-90). Majority of the patients were male (25 patients, 75.8%). The most common cause of acute respiratory failure were pneumonia followed by COPD acute exacerbation, acute pulmonary edema due to congestive heart failure, cancer progression, and lung fibrosis. The improvements were observed PaO2 and PaO2/FiO2 after 1 hr HFNC in comparison with baseline. 11 patients (33.3%) required endotracheal intubation, and the hospital mortality was 24.2%. The use of inotropics (p=0.04, odds ratio 0.17, 95% CI 0.033-0.914) and lactate (p=0.05, odds ratio 2.33, 95% CI 1.001-5.454) were associated with HFNC failure. Only lactate was associated with hospital mortality (p=0.037, odds ratio 1.49, 95% CI 0.255-1.933). Conclusion: Our results show a favorable effect of HNNC on clinical sign and oxygenation in ICU patients with acute respiratory failure. Serum lactate level could be simple indicator of HNFC failure and 28 days mortality.