Aim: The aim of this study was to detect the effect of dressing period of calcium hydroxide medicament on the apical marginal leakage after obturation of root canal system. Materials and Methods: 80 freshly extracted single rooted human teeth were selected ,all the root canals were hand-instrumented. They were randomly divided into four groups of 20 teeth each. Group A has no intracanal medicament , groups (B,C,D) were dressed with Ca(OH)2 paste for a different periods of time (7,14,30) days respectively, groups (B,C,D) were divided into subgroups of 10 teeth according to removing of dressing . The removal of ca(OH)2was done by two ways, 1- By irrigating with distilled water and drying with paper points . 2- Size 40 k-file was used in slightly filing motion up to the working length then irrigated with distilled water and drying with paper points. After that all groups were obturated with lateral compaction technique using gutta-percha points and zinc oxide-eugenol cement. Then the access was sealed with GIC cement. The teeth were immersed into 2% mythylene blue solution ,The specimens were placed in an incubator at 37 C ,with moisture at 100%, for 7 days .After that the roots were sectioned longitudinally and examined with stereomicroscope and liner leakage was measured.