Objective To evaluate the effects of root canal cleanliness on the fracture resistance of roots filled with AH-Plus. Methods Eighty single canal premolars were instrumented using step-back technique, then were randomly di-vided into four groups (n=20 for each group). Group A was washed with distilled water for 10 min, group B1 was washed with 5%EDTA for 1 min, group B2 was washed with 5%EDTA for 5 min and group B3 was washed with 5%EDTA for 10 min. Ten samples of each group were observed by scanning electron microscope at the coronal, middle and apical thirds to exam-ine smear layer removal. The remaining samples of each group were fixed into a electronic universal testing machine and ver-tically loaded until fracture. Results The difference of coronal and middle thirds was significant between group B3 and group B2 (P<0.05). At the middle third, there was significantly improved efficiency in smear layer removal in group B2 than that of group B1(P<0.05). The mean fracture resistance was significantly higher in group B3 (391.91±12.82)N than that of group B2 (335.54±16.14)N, group B1(296.47±17.82) N and group A (264.77±16.64)N (P<0.05). Group B2 showed a signifi-cantly better fracture resistance than that of group B1 and group A (P<0.05). Conclusion The complete removal of root ca-nal smear layer can significantly improve the fracture resistances of roots filled with AH-Plus.