Abstract： Objective To investigate the effect of immediate and delayed post space preparation on the integrity of the apical seal.Methods Seventy extracted human single-rooted human teeth were biomechanically prepared using the step-back technique.They were randomly assigned to 2 experimental groups of 35 teeth each and filled with two root canal sealants (AHplus and Cortisomol) separately.Thirty teeth in each group were prepared and the other five served as negative controls.Apical microleakage was evaluated with ink dye penetration.Results Cortisomol group showed better apical sealing effect than did other groups when teeth were prepared immediatlly,and microleakage values showed no statistically significant difference ( P ＞ 0.05 ),Vitapex group generated more microleakage than did the other Two groups with delayed preparation ( 2.03 ± 0.33 mm),the difference was statistically significant ( P ＜ 0.05 ).Cortisomol group had a greater microleakage than other groups did when prepared immediately,but the difference was not statistically significant.The other two groups showed significantly greater microleakage with delayed preparation than they did with immediate preparation group ( P ＜ 0.05 ).Conclusions Immediate post space preparation creates less microleakage than delayed preparation does.