Abstract: In this paper, Gleeble-1500, photomicroscope and TEM have been used to study the microstructures of the over-heated band of simulalcd heat-affected-zone (HAZ) of low carbon Si-Mn-Mo bainitic steel. The study is focused on the features of bainitic structures. Results show that typical structures of bainite can be eluded with the emerging of the so-called mcta-bainilcs, defined as the complex structure of bainieic ferrites and recained austenitcs instead of carbides in and/or beetwrn the ferrites. The formation mechanism of mcta-bainilc has also been examined.