Statement of Problem: Successful endodontic therapy depends on a well-adapted filling in conjunction with root canal. The proper restoration requires the placement of a post and core. Therefore, the time of post space preparation, and its possible effect on the quality of the apical seal is important.Purpose: The purpose of this study was to compare the effect of immediate and delayed post space preparations on apical seal using two currently used sealers; AH26 and Apexit sealers.Materials and method: Seventy six extracted anterior teeth were used in this experimental study. Teeth were randomly divided into four groups and all canals were prepared by step back method, and then filled with gutta percha by lateral condensation technique. AH26 was the sealer used in groups 1 and 2, while in groups 3 and 4 Apexit sealer was used. In groups 1 and 3, post spaces were immediately prepared. But in groups 2 and 4 they were prepared after one week. After preparation, the teeth were immersed in India ink for 3 days and finally, were cleaned. The extent of dye penetration was determined by stereomicroscope. The data were obtained and analyzed statistically with ANOVA, Tukey and T.tests.Results: The results showed that the lowest and highest rates of dye penetration were detected in groups 1 and 4 respectively. There was significant difference in apical leakage between the AH26 immediate, and Apexit delayed post space preparation.Conclusion: According to the results of this study, the immediate preparation of post space after obturation and before setting of the sealer is recommended.Key words:  Apical leakage, Post space preparation, sealer