Purpose: Postoperative atrial fibrillation (AF) is a common complication of cardiac surgery that is associated with an increased incidence of other complications. The goal of this prospective randomized study was to evaluate the effect of ultra-low dose landiolol hydrochloride for prevention of AF after off-pump coronary artery bypass grafting (CABG).Methods: The subjects were 47 patients who underwent isolated CABG and were randomly divided into those who received landiolol from ICU admission until the beginning of oral drug intake (Group L) and those administered diltiazem hydrochloride over the same period (Group D). The incidence of AF within one week after surgery was examined as the primary endpoint. Heart rate, blood pressure, cardiac output, and other hemodynamic parameters were used as secondary endpoints. The rates of adverse events were also recorded.Results: The incidences of AF in the first postoperative week were 4.8% and 27% in Groups L and D, respectively (p = 0.046). There were no differences in hemodynamic parameters between the Groups. In multivariate analysis, no factor emerged as a significant risk factor for postoperative AF. Two patients had adverse events of asthma and hypotension, respectively, in Group L.Conclusion: Ultra-low dose landiolol is effective for preventing AF after CABG without worsening hemodynamics.