Abstract: Objective: To investigate the cleaning capability of MTAN to the smear layer of post space walls after instrumentation. Methods: After screening, forty newly extracted premolars with single root canal were collected, and the crown parts of which were cut off above 2mm of enamelo-cemental junction. The root canals were shaped and obturated. Post space were prepared and divided randomly into 5 groups, and then irrigated by different solution: 10 ml of 0.9% saline (group A); 2.5 ml of 3% sodium hypochlorite followed by 2.5ml of 18% EDTA solution (group B); 5 ml of 3% sodium hypochlorite followed by 5ml of 18%EDTA solution (group C); 5ml of MTAN solution (group D); 10ml of MTAN solution (group E). All the samples were irrigated by 10 ml of saline ultimately. The morphology of post space was observed under field emission scanning electron microscope (FE-SEM), and scored by Peters’ criterion. Results: The smear layer existed in group A; a large amounts of thick smear layer were present and more dentin tubules were obturated in group B; the distribution of smear layer was up to 50% and the dentinal tubules were partly visible in group C; the smear layer was mostly removed in group D and E, and most of the dentinal tubules were open. Conclusion: MATN is effective to remove the smear layer after post space instrumentation.