Bacteria can invade into dentinal tubules that are wider than 2.5 micron. The size of OrthoMTA is smaller than 2 micron and can be infiltrated into dentinal tubles. After Orthograde canal filling, Interfacial layer is formed between OrthoMTA and canal wall. The composition of the interfacial layer is closed to hydroxy appatite. This layer was found to be bioactive and induce bioactive effect to around tissue. Bonding of this interfacial layer is mechanical and chemical bonding. As a result, perfect apical sealing is possible in orthograde canal filling with OrthoMTA.