Titanium microalloyed steel whose grain size is around 3μm has been developed by CSP process. Samples after different plastic deformation were obtained by suddenly stopping the six-stand finishing train during tandem rolling. Experimental results and analysis show that plastic deformation during tandem rolling plays a major role for grain refinement. With the successive severe strain, microstructure of the same stock during tandem rolling is further refined because of repeated recrystallization at higher temperature and strain accumulation at lower temperature. Technological features of CSP process, such as large reduction per pass, rapid cooling after rolling and high solidification and of thin slab, are important reasons for obtaining ultafine-grained structure. Besides these, chemical composition of titanium microalloyed steel has beneficial effects on grain refinement.