Introduction: The purpose of this study was to evaluate the influence of gamma-irradiation of teeth on the effectiveness of root canal irrigation and on the sealing ability of resin based sealers (A Comparative In-Vitro Study). Materials and Methods: in the first part of the study, A total of fifty recently extracted single rooted human permanent teeth were used and divided into 2 main groups according to exposure to irradiation: Group (I): Non radiotherapy group: those subjected to no irradiation (n=15). Group (II): Radiotherapy group: those subjected to a total dose of 60 gray of radiation (n=35). The samples of each group were then subdivided into 3 subgroups according to the irrigating solution used as final rinse: Subgroup A: Saline (Control group) (n=10, 5 for each group), Subgroup B: 17 % EDTA (n=5 for Group I, n=15 for Group II) and Subgroup C: MTAD (n=5 for Group I, n=15 for Group II). The teeth were decoronated and the root canals were instrumented using a step-back technique and irrigated. The teeth were then split longitudinally to be examined under SEM (X2000) for presence or absence of smear layer. The data were collected and statistically analyzed. In the second part of the study, fifty recently extracted single rooted human permanent teeth were collected. The teeth were decoronated and mechanically instrumented with a step-back technique. Fifty teeth were divided into 3 main groups according to exposure to irradiation: Group I: Non-radiotherapy group (n=10) the root canals of this group were root canal filled and not subjected to irradiation. Group II: Radiotherapy before root canal filling (RCF) group (n=20); the roots of this group were subjected to a total dose of 60 gray of irradiation before RCF Group III: Radiotherapy after RCF group (n=20); the roots of this group were subjected to a total dose of 60 gray of irradiation after. The samples of each group were subdivided into 2 subgroups according to the root canal sealer used for root canal filling: Subgroup A: (n=25) Gutta-percha and MetaSEAL root canal sealer. Subgroup B: (n=25) Gutta-percha and AH plus root canal sealer using lateral compaction technique. Evaluation of microleakage was done by using the silver nitrate dye penetration test. The data were collected and statistically analyzed. Results: part I: At the coronal and middle thirds of subgroup B and subgroup C the results revealed that: Group II had a statistically significantly higher mean rank of smear layer scores than Group I. While at the apical third, there was not a statistically significant difference in the mean rank of smear layer scores between both groups. part II: ANOVA test revealed that Group I had a statistically significantly (P=0.014) lower mean value of the dye penetration than Group II and a statistically non-significantly (P > 0.05) lower mean value of the dye penetration than Group III. Group II showed a statistically significantly (P=0.014) higher mean value of the dye penetration than Group III. Subgroup A had a statistically non-significantly (P>0.05) lower mean value (±SD) of the dye penetration than subgroup B. Conclusions: within the limitations of this in-vitro study, the following conclusions could be drawn: The irradiation dose used in this study had significantly reduced the debridement efficiency of the irrigating solutions used in this study at the coronal and middle thirds in addition it had a significant detrimental effect on sealing ability of ‘Gutta-percha/MetaSEAL and Gutta-percha/AH Plus’ to radicular dentin when the root canal filling procedure was carried out after exposure to irradiation and no significant alteration before exposure to irradiation was detected.