Abstract Aims Mitral isthmus (MI) ablation is challenging. We hoped to close those conduction breakthrough sites (CBS) across the MI by elaborate mapping. Methods and results After the initial linear ablation, elaborately mapping large areas above and below the MI line and inside the coronary sinus (CS) was sequentially performed to identify the CBS. The shortest distance from the CBS to the MI line was measured. The distant CBS (D-CBS) was identified as those CBS >5.0 mm away from the MI line. We prospectively enrolled 177 consecutive patients. Bidirectional conduction blockage across MI was obtained in 50 (28.2%) patients after the initial linear ablation and was achieved in additional 115 (65.0%) patients following elaborate mapping and reinforcement ablation. After initial linear ablation, 272 CBS (2.14 ± 0.99 CBS/person) were identified, and 226 (83.1%) of them were characterized as D-CBS, including 98 sites (36.0%) >10.0 mm and 39 sites (14.3%) >15.0 mm away. Endocardial and epicardial (CS) reinforcement ablation eliminated 119/272 (43.8%) and 58/272 (21.3%) CBS, respectively. Among the 177 eliminated CBS, 138 D-CBS (78.0%, 11.2 ± 5.6 mm) were confirmed in 95 (74.8%) patients. Moreover, CBS along the course of ligament of Marshall was closed by endocardial ablation more frequently than that along the course of great cardiac vein (52.6%% vs. 35.1%, P = 0.004). Eventually, CS ablation was required only in 64 (38.8%) patients. Conclusion Distant CBS, accounted for the majorities of the residual conduction across the MI after initial ablation, could be effectively identified and accurately eliminated by elaborate mapping and ablation around the MI ablation line.