Abstract: The morphologies and crystallographic features of bainite/austenite interface structure formed isothermally in an Fe-C-Si-Mn-Mo alloy have been studied by means of STEM(a Vacuum Generators HB5).These observations are interpreted in terms of current ideas concerning the structure migration characteristics of FCC/BCC interfaces.It is concluded that the bainite/austenite interface is probably closely similar to Widmanstatten ferrite described by Rigsbee and Aaronson. The differentia between bainite and lath martensite,Widmanstatten ferrite interface structure and migration mechanism, are discussed.