Aim: This study was directed to evaluate the efficacy of XP endo finisher file to reduce the bacterial count in root canals prepared to different apical sizes and tapers. Materials and method: Mesiobuccal root canals of extracted human mandibular first molar teeth were selected in this study and   prepared by using race rotary files into different apical sizes and tapers then the Xp endofinisher file used after these files. Results: A statistically significant difference in bacterial count was found between all groups after instrumentation where (p≤0.001). Also A statistically significant difference in bacterial count was found between all groups after using XPF where (p≤0.001). Conclusion: Using Xp endofinisher file after canal preparation with small apical size was more effective in intracanal bacterial reduction than larger sizes.