It is important to understand the difference of bainite transformation and eutectoid decomposition in steels. Microstructure of peariite and bainite were observed under JEM-2010 HREM and Quanta-400 SEM. The difference of bainite transformation eutectoid decomposition were discussed in nature and appearance based on present and previous work. The results show that bainite transformation isn't eutectoid decomposition, but take on transition character, new definition of Bainite in steels was given as well.