Introduction A novel real‐time lesion size index (LSI) that incorporates contact force (CF), time, and power has been developed for safe and effective catheter ablation. The optimal LSI was evaluated to eliminate gap formation during pulmonary vein isolation (PVI). Methods and Results Consecutive patients were enrolled, who underwent their first PVI using a fiber‐optic CF‐sensing catheter for atrial fibrillation between December 2016 and October 2017. The CF parameters, force‐time integral (FTI), and LSI for 3095 ablation points in 34 patients were evaluated. The FTI and LSI in the lesions with gaps or dormant conduction (gaps/DC) were significantly lower than those in the lesion without gaps/DC (FTI: 140.5 ± 54.5 and 232.4 ± 121.4 g s, P < 0.0001; LSI: 4.0 ± 0.6 and 4.7 ± 0.9, P < 0.0001, respectively). On receiver operating characteristic curve analysis, the optimal LSI threshold was 4.05 (sensitivity, 63.4%; specificity, 76.3%). The LSI of <5.25 predicted a gap or DC with a high sensitivity (sensitivity, 97.6%; specificity, 25.7%). In the posterior wall, which was 37% thinner than the nonposterior wall, a lower LSI of <3.95 showed a relatively high sensitivity (92.3%) and specificity (65.6%). Conclusions The LSI can be used to predict gaps/DC during the PVI procedure. An LSI of 5.2 may be a suitable target for effective lesion formation. An LSI of 4.0 may be acceptable in the posterior wall, especially in areas adjacent to the esophagus.