Abstract: Continuous cooling transformation (CCT) curves were plotted for the austenite before and after the compression deformation at 850°C, as a result of an investigation on a weathering steel which was tested on MMS-100 tester by means of dilatometry in combination with Vickers hardmeter. Optical microscope and SEM were employed to investigate the influences of cooling rate and deformation process on the transformation behavior and microstructure. Experimental results showed that in the whole deformation process the starting phase-transformation temperature increases with the polygonal ferrite area moving towards top left corner on the curve diagrams, thus increasing the critical cooling rate of ferrite plus pearlite. The deformed austenite suppresses the growth of bainitic ferrite by its tangled dislocation, then results in the refinement of microstructure.