The aim of this study was to compare penetration depth of sodium hypochlorite into dentinal tubules after passive ultrasonic agitation and ER; YAG activation.Materials and methods:Twenty-four single rooted human mature mandibular premolars were decoronated and accessed. After locating the apex and determining the working length, preparation of root canal was done up to #35 file using Mtwo system and with 5.25%NaOCl irrigation. Teeth were then sealed apically with wax and submerged in a crystal violet dye for 48 hours to stain dentin. NaOCl Irrigation was activated with either Ultrasonic or ER;YAG laser. Specimens were sectioned longitudinally and depth of bleached zone was evaluated under a stereomicroscope 40X.Results:Penetration depth was significantly higher in overall root canal in ultrasonic group than ER;YAG laser group (P=.000). In ER;YAG Laser group, the highest penetration depth was in the coronal third followed by middle and apical, with significant difference between apical third and both middle and coronal thirds (P=.009, .003 respectively), and no significant difference between middle and coronal thirds (P=.083).Highest penetration depth was seen in the middle third, followed by coronal and apical,withno significant difference in penetration depth between the three sections of the root canal activated with Ultrasonic (P=.664). Conclusion:ultrasonic activation can lead to more NaOCl penetration into dentinal tubules than activation with ER;YAG.