The purpose of this study was to evaluate the effect of immediate and delayed post space preparation on the apical sealing ability of EndoREZ, a methacrylate-based dual-cured resin sealer, with or without accelerator. Fifty extracted human teeth were endodontically prepared and randomly divided into 6 groups. Teeth in experimental groups 1, 2, 3, and 4 (n = 10) were filled with resin-coated gutta-percha and either EndoREZ with accelerator (A+) or EndoREZ without accelerator (A−) by the lateral compaction technique. In group1 (A+) and group2 (A−), the post space was prepared immediately at the time of obturation. In groups 3 (A+) and 4 (A−), the post space was prepared after storage in 100% humidity at 37°C for 1 week. Groups 5 and 6 (n = 5) represented positive and negative control groups, respectively. Leakage was determined by computerized fluid filtration device. Complete leakage was observed in group 5, and no leakage was evident in group 6 (P = 1.000). In experimental groups, mean apical microleakage values (μL/cmH2O/min−1) were as follows: group 1, 2.77 ± 0.79a; group 2, 2.88 ± 1.38a; group 3, 19.95 ± 7.85c; group 4, 6.20 ± 2.09b (different letters indicate significantly different groups, P < .005). Immediate post space preparation achieved better sealing than delayed post preparation at the apical end, regardless of whether the EndoREZ accelerator was used. There was no significant difference between the (A+) and (A−) in immediate post space preparation. However, more leakage was found when using (A+) than (A−) in delayed post preparation. Under the conditions of this study, it was concluded that EndoREZ accelerator seems not to provide clinicians with any advantage for rapid transition from endodontic treatment to post-endodontic restorative procedures immediately after completion of root canal therapy.