Objective  To investigate the effect of coronal microleakage of cemented cast posts and cores after different interval time preparation. Methods  Cast posts and

cores were fabricated for 60 of 66 extracted single-root mandibular premolars which randomly divided into 6 groups and cemented with zinc phosphate cement. The other 6 specimens acted as negative control group and positive control group. After 2, 10, 30 minutes interval time, preparation would do for preparation groups and three control groups without preparation. After dying and making transparent, the assessment of the coronal microleakage was done. Results  The results showed significant differences between each prepared group and unprepared group. No differences were defected in 3 unprepared groups. But significant differences were recorded between after 2, 10 minutes preparation groups and 30 minutes group. Conclusion  High-speed preparation had a significant negative effect on the coronal microleakage of posts. The interval time of preparation after cemented cast posts and cores was at least between 10 and 30 minutes.