Abstract: In-situ observation for super-cooled austenite microstructure transformation of the strip casting low carbon steel was performed by confocal laser scanning microscopy. The results show that the average grains size of original austenite grains in strip casting low carbon steel was about 200 μm, the room temperature microstructure was mainly composed of bainite and polygon ferrite. The main nucleation positions of bainitic ferrite plats (BF) were original austenite grain boundary, inclusions in austenite transgranular and grain boundary polygon ferrite or grown BF plats during super-cooled autensite microstructure transformation, the first nucleation BF plate did not across the secondary nucleation BF plate, and the former and the latter took a certain orientation angle. The temperature, nucleation position and growth orientation had effects on the growth speed of BF plate.