Objective: It is aimed to compare the effect of root canal treatment and retreatment procedures on the fracture strength of roots. Material and Methods: One hundred and twenty extracted single-canal mandibular premolar teeth were involved in the present study. The bucco-lingual and mesio-distal diameters of the roots were measured. The crowns of teeth were removed from the point of cemento-enamel junction in the way ensuring the standard root length of 14 mm. The teeth were then randomly divided into 6 groups (n=20). In Group 1, no instrumentation or obturation was performed. The rest of the roots were chemo-mechanically prepared with nickel-titanium rotary instruments (ProTaper Next, Dentsply Maillefer, Ballaigues, Switzerland up to size 40/.06. The teeth in Group 2 were left without obturation. Group 3 was obturated by using the warm vertical compaction technique. Those in Group 4 were obturated and retreated (20/.07). The teeth in Group 5 were obturated, retreated and apically enlarged (50/.05). Finally, the teeth in Group 6 were obturated, retreated, apically enlarged (50/.05) and re-obturated and then the fracture resistances of the all specimens were tested by using a universal testing machine. Results: There was no statistically significant difference between the fracture resistance of teeth in Group 1, Group 3, and the Group 6. No statistically significant difference was found between the teeth in Group 2, Group 4 and Group 5. The lowest resistance to fracture was found in Group 5. There were statistically significant differences between Group 1, Group 3, Group 6 and Group 2, Group 4, Group 5. Conclusion: Obturation after preparation and retreatment procedures increased the resistance of teeth to fracture.