Introduction: Single-shot ablation has emerged as an effective technique for index atrial fibrillation (AF) ablation, with an advantage of short procedure time. Although recent guidelines recommend peri-procedural uninterrupted oral anticoagulants (OACs), the intra-procedural anticoagulation strategy remains uncertain under non-vitamin K OACs (NOACs). We investigated procedural safety of a single bolus administration of heparin without activated clotting time (ACT) measurement during cryoballoon ablation (CBA). Methods: Two hundred patients (64.2±10.0years, 70% with non-paroxysmal AF) who underwent CBA with uninterrupted NOACs were randomly assigned to No-ACT group and ACT group. A bolus of heparin (100 U/kg) was routinely administered immediately after transseptal puncture. In the ACT group, an additional injection of heparin (30 U/kg) was administered if ACT at 30-min after the initial bolus was <300 s. Results: There were no differences in baseline characteristics including CHA2DS2-VASc score between two groups. The left atrium indwelling and procedure times were 60.4±13.1 min and 78.9±13.9 min, respectively and not significantly different between two groups. The mean ACT was 335.2±59.9 s in the ACT group. Any bleeding rate was 3.2% in all patients and there was no statistically difference in bleeding complications between two groups. In the ACT group, groin hematoma, laryngopharyngeal bleeding, and hemoptysis occurred in 3, 1, and 1 patient, respectively. Cardiac tamponade occurred in 1 patient in the No-ACT group. No thromboembolic events occurred during the 30-day follow-up after CBA. Conclusion: Single bolus administration of heparin without ACT measurement is a feasible anticoagulation strategy for CBA in patients with uninterrupted NOACs intake.