Objective: To investigate apical sealing ability of warm vertical compaction in different tapered canals (taper 0.06 and taper 0.10) in mandibular molars. Methods: Thirty extracted, single-mesial root with two orifices and two mature apices human mandibular molars were used. Both canals of each mesial root were instrumented to size 30 with different tapers (taper 0.06 and taper 0.10). Then all canals were obturated with warm vertical compaction technique. Dye penetration and clearing method were used to assess the apical sealing ability. Results: Mean microleakage of taper 0.06 canals was 1.20 ± 0.70 mm. Mean microleakage of taper 0.10 canals was 0.43 ± 0.29 mm (P < 0.01). Eight cases of extrusion were found in taper 0.06 canals, while one was in taper 0.10 canals (P < 0.05). Conclusion: Taper 0.10 is a better choice when warm vertical compaction technique is used in mandibular molars.