Introduction & Objective: The success of root canal therapy depends on the ability to establish and maintain an apical seal, therefore, it is important, that post space preparation do not disrupt the integrity of the apical seal. The purpose of this study was to compare the effect of immediate versus delayed post space preparation on the apical seal using AH26 and Dorifil sealers.

Materials & Methods: In this experimental study forty six extracted premolars intact, single canal, and caries free were used. Teeth were randomly assigned to 2 groups and three teeth assigned to each positive and negative control group. Teeth were prepared with step back method and filled with gutta-percha and sealer by lateral condensation. In groups 1 and 2 AH26 sealer and Dorifil sealer were used respectively. Post spaces were made until 5mm of the apex after placing the teeth in incubator for one week. The teeth were attached to bacterial leakage assay system. Staphilococus epidermidis was placed every 48 hours into the root canal of the teeth. Duration of study was 60 days. The data was statistically analysed with chi square and t-test.

Results: In AH26 group 19 samples and in Dorifil group 18 samples were leaked. The mean time of leakage in AH26 group and Dorifil group were 22.58± 11.37 and 25.94± 1058, respectively. The results showed that there were no significant differences in incidence and mean time of leakage between AH26 and Dorifil groups (P= 0.54 and P= 0.35), respectively.

Conclusion: Under the condition of this study, teeth will be contaminated after post space preparation if access cavity are open. No significant differences were found in the AH26 and Dorifil sealers groups.