Introduction Although left atrial posterior wall isolation (LAPWI) in addition to pulmonary vein isolation is a well-accepted option for persistent atrial fibrillation (AF), complete isolation can be challenging. To evaluate performance of a modified ablation index (AI) (AI/bipolar voltage along the ablation line) for predicting durable LAPWI. Methods The study involved 55 consecutive patients, aged 65 ± 11 years, who underwent electroanatomic mapping-guided LAPWI for AF. Association between gaps (first-pass LAPWI failure and/or acute LAPW reconnections), voltage amplitude along the roof and floor lines, and thickness of the LAPW was investigated. Results Gaps occurred in 22 patients (40%) and in 26 (8%) of the 330 line segments assessed—11 in the center roof line segment, 6 in the center floor line segment, 4 in the right roof line segment, 4 in the right floor line segment, and 1 in the left floor line segment. Gaps were associated with relatively high bipolar voltage (3.38 ± 1.83 vs. 1.70 ± 1.12 mV, P < 0.0001) and a thick LA wall (2.52 ± 1.15 vs. 1.42 ± 0.44 mm, P < 0.0001). A modified AI ≤ 199 AU/mV, bipolar voltage ≥ 2.64 mV, wall thickness ≥ 2.04 mm, and roof ablation line ≥ 43.4 mm well predicted gaps (AUCs: 0.783, 0.787, 0.858, and 0.752, respectively). Conclusions High voltage zones, a thick LAPW, and a long roof ablation line appear to be determinants of gaps, and a modified AI ≥ 199 AU/mV along the ablation lines appears to predict acute durable LAPWI.