Background: The space for irrigant cleansing action and ultimate root canal dimensions are influenced by root canal size and taper. Aim of the study: to investigate the effect of minimal invasive endodontic preparation technique with different irrigation protocols on the cleanliness of extracted mandibular permanent molars. Materials and methods: A total of eighty extracted mandibular permanent molars were selected in this study. All teeth were divided into 4 groups depending on the taper and size of the file that was used for each group: group I: up to size 25 taper 0.04, group II: up to size 25 taper 0.06, group III: up to size 30 taper 0.04 and group IV: up to size 30 taper 0.06. Each group was randomly divided into two subgroups: In Subgroup A: traditional irrigation technique without activation of the irrigant while in Subgroup B: traditional irrigation technique with ultrasonic activation of the irrigant. Results: In this study, when comparing the subgroup A to subgroup B of each group, it revealed significant difference between them regarding group I, group II and group III, but no significant difference between them regarding group IV in case of debris removal. But, it revealed no significant difference between them regarding smear layer removal except in group I. Conclusion: A root canal preparation to a size 25 taper 0.04 with irrigant activation resulted in significantly less residual debris in the root. In addition to, there is no significant difference in smear layer removal between all the groups.