Inhaled nitric oxide( iNO) therapy is commonly used to improve pulmonary hypertension and oxygenation in adult patients undergoing open heart surgery, mostly being applied to mechanical ventilation (MV). We often face rebound of pulmonary artery pressure (PAP) after reduction or discontinuation of iNO therapy, resulting in prolonged MV. Twenty-three cases, to which iNO therapy during MV (MV-iNO) were initiated, then continuously treated with iNO therapy using high-flow nasal cannula (HFNC-iNO) after extubation, were retrospectively investigated. During MV-iNO, mean PAP( mPAP) was significantly lower than before starting iNO therapy (p<0.001). Also, mPAP on HFNC-iNO was significantly lower than mPAP before iNO therapy during MV (p<0.001). There was no significant difference of mPAP between MV-iNO and HFNC-iNO (p=0.38). MV was discontinued in 330 minutes (median), oxygenation was maintained after switching from MV-iNO to HFNC-iNO and there were no cases of reintubation, perioperative mortality, or adverse events due to iNO therapy. HFNC-iNO is considered as useful method in maintaining decreased mPAP and improved oxygenation after extubation in adult patients after open heart surgery.