Objective: The study was aimed to evaluate the Effect of cryoirrigation using Endovac and needle irrigation on reduction of the external root surface temperature of the mesial roots of mandibular first molars with different root canal configurations. Methods: Out of 153 mature mandibular first molars, 20 teeth were selected. The selected teeth, 10 of them had mesiobuccal roots with type II root canal systems and 10 of them had mesiobuccal roots type III root canal systems according to weine classification. The teeth were divided according to the temperature of the irrigant used into two main groups, (A1), room temperature and, (A2) 2.5°C. Each group was further subdivided according to the anatomy of the root canal being irrigated into two more groups, (B1) root canals with one portal of exit (type II root canal system) and (B2) root canals with two portals of exits (type III root canal system). Finally, each subgroup was further subdivided according to the technique of irrigation used into two subgroups: (C1) Needle irrigation and (C2) Endovac irrigation. Access cavity was accomplished, root canal preparation was done using rotary NiTi files to a master apical size of #25 taper 0.06., cryo irrigation was done and the external root surface temperature was measured using type K thermometer attached to thermocouple. Result: There was a significant difference in external root surface temperature between cryoirrigation groups and room temperature irrigation groups. Also, there was a significant difference in cryoirrigation groups between Endovac and needle irrigation techniques. Conclusion: cryoirrigation decreased external root surface temperature.