It is important to establish effective treatment for postoperative atrial fibrillation (AF), the most common complication after cardiac surgery. We evaluated the efficacy and safety of landiolol hydrochloride for rhythm conversion in patients with postoperative AF. Among 134 patients who developed new-onset AF after open heart surgery between 2007 and 2009, 69 patients who received landiolol hydrochloride for treatment of postoperative AF were enrolled. The AF conversion rate, the percentage of patients with 20 % reduction of the ventricular rate, and the factors related to successful treatment were evaluated. Then, the landiolol group was compared with 65 patients who had postoperative AF and did not receive landiolol hydrochloride. Landiolol hydrochloride was the first-line treatment in 46 patients and the only therapy in 26 patients. Reversion to sinus rhythm was achieved in 51 patients, while the conversion rate in patients without landiolol hydrochloride was only 56.8 % (p < 0.05). A 20 % reduction of the ventricular rate was achieved more frequently in the landiolol group. Although landiolol hydrochloride was highly effective in patients who had undergone off-pump coronary artery bypass grafting, patients with cardiopulmonary bypass did not respond as well. The heart rate was reduced from 130 ± 26 to 81 ± 12 (p < 0.05) after landiolol administration, while blood pressure did not decrease significantly. Landiolol hydrochloride was effective for conversion of postoperative AF. This ultra-short-acting β-blocker is a safe first-line treatment for postoperative AF after open heart surgery, and is most effective in patients who have undergone off-pump coronary artery bypass grafting.