The aim of this study was to compare vertical forces at fracture of endodontically treated mandibular incisors obturated with different types of root canal sealer. Four groups of 10 teeth each were tested; group 1 served as positive and negative controls (five teeth each), and groups 2, 3, and 4 were obturated by lateral condensation with gutta-percha and AH Plus, Tubliseal, or Ketac-Endo, respectively. All teeth were loaded vertically using a plugger tip inserted into the canal space until fracture occurred; the load at fracture and the pattern of fracture were recorded. Statistical analysis was performed using a one-way analysis of variance and post hoc Scheffé test at the 95% level of confidence. Force at fracture of roots obturated with Ketac-Endo was significantly higher than those obturated with AH Plus and Tubliseal. Most fracture lines were in a buccolingual direction. The results suggested that Ketac-Endo strengthens endodontically treated roots and may be used for weak roots, which are likely to be susceptible to vertical root fracture.