Purpose: The aim of this study was directed to compare the sealing ability of two materials which are conventional zinc oxide and zinc oxide nanoparticles by measuring microleakage of both materials by stereo microscope and scanning electron microscope. Materials and Methods: Seventy extracted single-rooted human teeth with complete roots, fully formed apices and present a single canal without calcification were used. The crowns of all teeth were removed using a water-cooled, slow-speed diamond precision disc, so as to adjust the length of the roots to a standardized length of 16 mm. The samples were randomly divided into two main groups each one contains 35 samples according to the sealer used as follow: Group I: Conventional zinc oxide sealer & Group II: Zinc oxide nano-particles. Each main group was divided according to evaluation method as follow: Thirty roots were randomly selected from each group for the evaluation of sealing ability using dye penetration method to measure microleakage using stereo microscope and the rest (5 roots) were used in the SEM for representative evaluation of sealing ability of each sealer. In the stereo microscope part, the samples were further divided into three subdivisions according to the time of incubation period one week, one month and three months. Results: As for Dye leakage results of the sealing ability, zno nano-powder showed the statistically significantly highest sealing throughout all periods of examination, While conventional zno showed the statistically significantly lowest sealing throughout all periods of examination. For both zno nano sealer and conventional zinc oxide samples, the highest mean value of leakage was found in (After 3 months) followed by (After 1 week) while the least mean value of leakage was found in (After 1 month). Conclusion: Zinc Oxide nano particles based sealer has higher sealing ability than conventional zinc oxide based sealer.Nano sized particles allow more adaptation and penetration into dentinal tubules than macro sized one.